I can run a 5k around 19 minutes. However my average speed for a 15 mile stretch of road without interruptions is around 17mph. There are no significant hills. The entire route is fairly flat.

On the other hand I know a 14 year old kid who can ride at 19 mph or sometimes 20 mph pace for the same distance. He cant even run a 5k under 25 minutes. He is faster than me on a bike but slower than me on runs.

Then you have a woman in her mid 30s that averages 22 mph. Granted she has been riding for a long time, I still beat her on the 5k. She runs her 5k around 23 minutes. I have over 4 minutes on her when it comes to running. But she is that much faster than me on a bike???

The question for anyone that might know, is the following; why am I slower on a bike compared to people that I would totally annihilate on a run?

I ride 30 to 50 mile rides every other day. Ive been at this pace for roughly two years now with not much improvement. The fastest I ever averaged a flat stretch of road that was 15 miles distance without interruptions was 18 mph and that doesnt happen every day. I did however improve since I started riding seriously 5 years ago. Back then I was averaging roughly 15 mph. Am I slow on improving or is this average for someone of my experience?

Im also not an avid runner. I run maybe once or twice a week.

In order to improve you need to continually adjust your training to get a training response.

By this I mean that what was difficult for you two years ago is no longer difficult now. Your body has become more efficient and so, in reality, you are stressing it less by continuing to do the same work outs at the same level of intensity.

People who always ride the same route will often fall victim to this.

You need to mix up your riding and join a group so that others will push you beyond your comfort zone.

Physiology also plays a factor. Perhaps you are better built for running but cycling on flat roads isn't your strongest suit on the bike. Maybe you would be a better climber?

Big, powerful riders will have higher ave speeds than you on flat roads but, when the road tilts up, smaller guys with a good power to weight ratio will come to the fore.

I hope that gives you some basic information to start with.

Im bigger than both of the people I mentioned by atleast 40lbs. Im not a big guy but Im not small either. Im not a climber. Most of my rides are climbs. I live nearby mountains and thats all I do because I enjoy going downhill at 40 to 50 mph. Im slow to climb. Anything over 7% grade will have me going at 6 or 7mph. I have decent acceleration but Ive seen other people out accelerate me. Im not a sprinter because I can sprint till I explode and I am still going 34 mph at best for only 3 or 4 seconds. I get passed a lot on bike paths by riders doing over 20 mph.

Just riding around will not make you faster. Just as jogging will not make you a faster runner. And descending is a skill, but it won't help your sprinting, climbing. If you are a runner and want to get faster, you do intervals that take you anaerobic, and eventually your gain speed (simply put). In order to get faster on a bike, you don't ride more miles. You ride faster miles (In short bursts gradually lengthening the bursts)and to do that you have to take yourself above your current speed to get the "growth".
As Bob said, the best and easiest way to get quicker is to ride with people that ride more quickly than you. You will struggle at first, then gain speed.
And ask any triathlete, running and cycling use muscles differently. I ride with a couple of runners who are in great aerobic shape who struggle some on a bike. Mostly because they dont know how to "gear" the bike correctly...trying to teach them. But their legs are really lean, not powerful like a cyclists. So we are trying to find a happy medium between running and faster cycling.

Hollow, You've raised an question that has been crossing my mind of late. Next year I'm gonna do my first triathlon (400m, 23k, 5k) along with some colleagues. One colleague is both a good runner and a fair swimmer. Looking forward I expect my colleague to be out the pool some 5 mins before me but I expect to catch and pass him on the the bike finishing some 5 mins + ahead which leaves the run. He'll do the run in under 25mins I'll be 30mins +.

Could be a close finish!

Mostly.

correct

But not all. RW is correct. Intervals will make you faster and help you achieve whatever pathetic, laughable, lame and wannabe potential you have.

God we all do suck at this.

Mostly.

Blame it on the bike. That's what I would do.

No Cervelo for you. Until Cervelo makes recumbents.

God forbid.

If you were to try and do the course at >17mph (like 20mph or something), what would eventually make you slow down? Would you be out of breath or would your legs mostly be hurting too much? If it's your legs hurting, you could try riding with a faster cadence.

Running != Cycling

If you are a good runner it will give you a nice aerobic base and that is pretty much it.

One theory: Cycling is a different kind of pain. Until you're comfortable with it, it'll keep you in check.

Genetics and training.
Get with a faster group and hang on to them till you can stay up.

Mostly drugs

Training improves everyone but my boss can kick my ass even if we trade bikes.

cycling will improve your running, running will not improve your cycling

I've recently discovered that you tend to use different muscle groups for running and cycling. When much younger, I ran on the cross-country team and could run 5k in the15 minute range. Fast forward many years and I've biked many miles and run very few. This winter, I've been doing some training for a triathlon next august, and my running is stinking the joint up. I am barely able to keep 7 minute miles going, but it isn't my aerobic fitness that's limiting, it is the running specific muscles.

Genetics do help, but training is huge.

It took me YEARS to get my 5k from 21ish to 18ish. I didn't make the breakthrough (15 year plateau) until I ran nearly 80-90 miles per week, or over 12-15 miles per day at peak. That's big time training.

Unlike you though, I hopped on a bike and was immediately riding 22mph and hanging with Cat4-5 roadies. On my very first try. But despite this fast nature, it took a fair amount of training to crank it up another notch, and I'm not cycling anywhere near what I should be in terms of volume.

In my experience, running gives you a huge aerobic base, but doesn't necessariliy give you the leg power to ride fast or climb hard. I was lucky in that I have natural leg power - my quads and glutes are naturally big, AND I built power as a teenager through basketball and soccer. But that probably costs me in running, where small legs and small quads are much more advantageous for distance running.

I still think that for non-pro racers, training trumps all. All the way up to the top amateur levels. It may take you 7-10 years of highly disciplined training to get there, but it's doable. Going pro though, is a whole different matter, and you definitely need genetics AND training to be in that playing field.

i can barely run. but i'm not too bad in terms of stamina on the bike.

Not correct. It totally depends what kind of cycling and running you are doing. If you are rec cyclist that does a couple easy rides a week but you do some brutal run work outs above threshold that will translate into some improvement on the bike. I am not saying this is the way to train but I think you are underestimating the value of running particularly to cyclist that dont know how or like to train hard (eg. my wife). My wife does not like to put in hard efforts on the bike but she plays a lot of soccer, which can equate to a decent run interval workout. Playing soccer allows her to be a faster rider that she would be otherwise.

My impression, based soley upon what hurts afterwards, is that running uses more muscles than riding. Furthermore, unless you are getting into the upper 20 mph's, running is harder than cycling. So it makes sense to me that a conditioned runner would defeat a cyclist at running. I also think that it takes a little longer to develope cycling prowess than running.

In a word, YES. Genetics is huge but so is training. You could be genetically superior but without proper training your just going to be average. One odd thing though, Eddie Mercx hadn't rode a bike in 20 years, gained some mid girth weight and got on it about two or three years ago and went for a ride averaging 21mph for an hour leaving far younger journalists gasping trying to keep up and most gave up! Obviously his genetics are superior to most. So if your not gifted genetically but train smart and really hard you could do reasonably well and even win local events since most, if not all the riders in those type of events would not be genetically gifted as well.

Personally to be successful at pro level racing in any physical sport, genetics is more important then training as your base to work from. By the way politics is also very important to get into pro level anything.

Running a flat 5k (not a hill climb) predominantly involves your pulling and lifting muscles (hamstrings and hip flexors, respectively..calves and ankles doing much of the pushing), riding always requires "pushing muscles" predominantly (quadriceps, while calves and ankles minimally involved).

I am a **** runner (7:30 pace for 5k, best 1 mile = 6:15), but I've had some success at rowing (all pushing with the lower body, pulling with the upper body) and would not have a hard time averaging 22+mph for 15 flat miles on the bike. I've also spent minimal time running, but having trained for so long for rowing, it lends itself more toward cycling than running.