You're more or less at your prime, which does not jibe at all with the results below:
There's something massively wrong here, have you checked that your brakes aren't rubbing, your watch is not running at double speed, and that your cyclocomputer is set to the proper wheel size?

I'm measuring the distance with Google Earth after I ride and I'm pretty sure the watch is right. These are wide, knobby tires.

Brakes? Um . . . yep, it seems the back brake is rubbing some. I'll have to fix that and see what kind of difference it makes.

It should be quite a big one, depending on how much the brake was rubbing

My two cents...

The difference between a walmart MTB and a decent roadie (or decent MTB) will be mostly the amount of energy you are using. You may see a slight gain in speed, but mostly it will feel easier to go fast. The amount of energy lost to the drive chain is significant, even on $5k bikes. If you've ever had a chance to try a decent fixed gear, you'll see what I mean. The shoddy components on a wal-mart special are just going to exacerbate those losses more than is necessary. That'll kinda make it feel like you're wearing bricks on the end of your feet. One other big loss is the silly mock suspension things they have on sale at walmart. They've got the resistance on the shocks so low that you spend almost as much energy bouncing up and down as going forward. If you have one of those, then the best thing you can do is to get something else as soon as possible, even if it's just a beater you got at a garage sale for $30.

Also, to improve your time right now, adjust your seat and watch your cadence. Most people I see ride with the seat so low their knees are in the way of the handlebars. You should raise it until you only have a slight bend in your knee at the bottom of the pedal stroke. That'll give you better leverage and body mechanics and probably make you hurt less. You also should be completing about one and a half full rotations of the pedals every second. That's a ball park figure, but it will make your energy output lower for the same speed. Lowest reasonable cadence is about 60 rpm, or 1 rotation per second.

wut

Drivetrain losses on chain-driven bikes are about 1-2%

Rolling resistance from wide knobby tires and aerodynamic drag from an upright position are going to waste far more energy than extra drivetrain losses with shoddy components.

when I weighed 240 starting out riding last year I would do 9 miles in 56 minutes using a 40 lb Iron Horse.... 2 months later on a Schwinn Peloton road bike I would do 10 in 45. I can't quantify the road bike itself helped more than the added 2 months, but I remember the placebo effect of the road bikes feel. With its stiffness, lightness and overall stability giving me the ambition to try harder for longer.

I rode a wal-mart bike for about a year back in the day. Seemed like I couldn't go more than a day or two without hearing a new noise, rubbing, squeaking, etc. coming from the drive train. I'm willing to bet a lot of energy was lost that way. It also felt like I was dragging the chain around it's circuit rather than driving the bike forward.

On fairly efficient bikes, yes. I'm willing to bet that friction on various parts of the drive chain will raise that to 5-7% on a walmart bike. Pretty significant, if not a life changer.

Yes, but that's already been covered pretty well in this thread.

+1 90% of changing to a road bike is the experience.

Make up your mind. We were talking about "The amount of energy lost to the drive chain is significant, even on $5k bikes".

I was making the point that there is some inescapable energy loss there, but cheaper components make it worse. Like I said, try a fixed gear that is set up correctly, and the energy loss drops enough to be noticeable. My bad for getting off topic in the middle of a paragraph, but geez, little jumpy are we?

it's not much of a loss. I can verify that with powertaps and SRM.

I bought a Costco Schwinn before that Iron Horse... I can't even consider those stats to have merit as the bike only lasted me 2 weeks before the wheels flexed from my weight and paltry torque to the point of rubbing the inner chain-stay every time I utilized the right crank arm.

..speaking of which... does that Walmart bike have a suspension system? That would suck ass too.

You were saying that it's still pretty significant even on $5k bikes and I'm saying it's not.

Yes. And yes it is annoying, especially going up a hill.

Adjusted the brakes. Adjusted the seat. Pedaled harder.

8 miles in 48 minutes. Still in last place, but maybe a few people stuck around to watch me finish.

It is foolish to assume things. I recall one female who I saw climbing who was in way to high a gear and was going up and dwon like somethign on a merry-go-round.

When I first started riding my first bike was a mountian bike, fixed frame and pretty decent bike. There was more than the 3 mph difference other mention comparing that with slicks to my road bike. BUT I was a bit of a grinder and I easily was topped out on gearing once the slicks were on it.

If the OPs bike is a cheap suspended mountian bike with knobbies at mountian preasure and resonably poor drivetrain lubrication 5 or MPH difference could be reasonable, especially if he is going at low RPMs and betting the suspension going.

Are you saying I am being foolish or are you in agreement with me?

My thought too. FWIW, I think 9 mph on a cheap 35 lb MTB should translate to 11-12 mph on a good-quality used 24 lb steel/alu roadbike. Maybe more in a race, your adrenaline gets pumping, your putting out more watts than just riding by yourself, tooling around. That's what being in a race does to your brain and bod.

This being said, OP needs to get into better shape, and if he can do 20 mi/day 5-6 days a week for 6 weeks, and push himself, not to puke levels, but to "I am really bushed," the speed is going to increase.

Bottom-tier bikes suck. They are made for 9 mph, "I'm riding the MUP, if the weather is 75 degrees; I'm not really interested in committing myself to this cycling thing," easy-pace weekend riders. OP's WM bike is not made for any kind of race, XC trail or road. A good raceable mid-range MTB is 24-26 pounds, not 35.

Any roadbike will just feel more responsive when you crank it, and make you want to crank harder and faster. I could see OP doing 15 mph , with 6 weeks getting into better shape from consistent diligent daily riding, and with the adrenaline flowing on race day.

OP, i think the biggest thing you would notice if you switch to another bike is the lack of suspension. Even if you drop $300+ for a brand new entry-level, but name brand mountain bike (not particularly recommending it, just an example) you'll notice a big improvement from lack of squishiness. That's what I used to hate about my walmart beater growing up. I couldn't spin up to any real speed without bouncing up and down like a jack-in-the-box. Like I said, they make the shocks way too loose, probably so people will notice it over little bumps on the sidewalk.

6 weeks is a little bit too short a time to reliably hunt down a good deal on craigslist or ebay, but I would recommend that route. You might be able to find a good deal on an older steel road bike for ~$100. Make sure it fits, and it might be a good idea to get to know your local bike shop. Their advice will be helpful, and you might like for them to do some work you are not comfortable doing. Mine at least does tune-ups for pretty cheap.

If all else fails, switch over to slick tires. Sure it won't make the biggest of changes, but you can usually get some decent slicks from performancebike.com or nashbar.com for $30 for a pair ($14.99 or something each) on sale. Now my level of decent is different than most people on here's, but it's probably the cheapest change you can make if you can't find a yard sale bike that fits.

It's really starting to get off topic now, so this is the last time I'll post on this, but even 1-2% is significant. 1% loss is the difference between 25 mph and 24.75 mph. Lots of pro's spend time in a wind tunnel trying to gain a lot less than that in aerodynamic advantage.

Yes I agree that when racing 1% can be significant, but it was really in the context of the comparison to fixed gear bikes and low end bikes with "shoddy components".

thats also 16mph vs. 15.68mph. I guess I'm slow and inconsistent. I'd call that a normal variance, depending on how hydrated he is and what he had for dinner the night before, whether the tire pressures were off by a little bit?

Sorry, didn't mean to sound like a dick. Wasn't my intention.

I just meant that you're doing something wrong. Your time should be so much faster that that. If you can run 5 miles in 50 minutes, you're pretty healthy.

Forget the bike for now - just ride it for as many hours per week as you can - and focus on the run leg.