Welcome to the addiction Scott!
Originally Posted by WantsToRace
A few newbie questions to help me understand things:
1) If I can continue making gains and get the pace up to say 18 MPH on the Mongoose, how much speed could I realistically expect to gain by switching to a nice lightweight bike, with no other changes? I've heard 2 or 3 MPH average - is this accurate?
It really depends upon what kind of bike you have now. The benefits of lighter weight varies by terrain and makes the most difference on the hill. On the flats, speed is limited more by aerodynamics than weight. Have your body-position checked out and practice riding in the hooks (drops) of your bars to stay low with a flat back. I like to check my position when riding past large grocery-store windows and keep my back flat and low with the side-panels on the jersey parallel to the ground. This gives way more improvements in speed than any amount of weight savings, but it does take months of training to get used to it.
Originally Posted by WantsToRace
2) Once I'm on a lighter, better bike with smaller tires, if my neighbor and I start alternating drafting each other, how much speed should we expect to pick up then?
Drafting allows you to save energy at any given speed. Won't really help much as far as increasing speed, you really need more than 2 people to see significant improvements in speed. With 2 people, you can pull hard for 1-2 minutes and trade to rest. This will let you increase your average speed by maybe 2-4 mph.
Originally Posted by WantsToRace
3) For a typical 20-mile race, what average speed is required to be fairly competitive? And just out of curiosity what average speed would the first place winner be doing?
It really comes down to the course and mix of riders in the pack. The winner will have exactly the same average-speed as the last-place finisher; they usually finish less than 100m apart (stragglers usually get pulled from the race). If you can average 18mph on your own, then that should be fine for hanging with the pack. If you can average 20mph, you're competitive enough to win a race. But racing is more about strategy and playing chess than pure fitness. Pretty much anyone can hang in the pack and finish. They average about 24-28mph for a cat-5/cat-4 race and the draft will let you hang no problem if you can average 18-20mph on your own. HOWEVER, there are spurts reading 35mph and the sprints can come close to 40mph, so you need to have that kind of top-speed capability or else you'll get dropped.
In order to deal with this, you need to now diverge your training in to very specialised workouts. You need to do more distance once a week, like 2-3 hours of 30-40miles at an EASY pace, slow down! I suspect most of your rides are way, way too fast and you really do need to do more distance at an easier pace. Work up to 3-4 hour rides of 40-60miles at an easy pace that you can hold the entire time. Eat and drink lots. This trains your energy system to be more efficient and burn more fats. This then makes your higher-pace efforts more energy-efficient as well.
Then, and only then, start picking up the pace more. And you need to really pick it up more than you're doing. You'll need 1 day of sprint workouts, 35-40mph of no more than 10-15miles total on the ride. These are 100% all-out, screaming-bloody-murder sprints as hard as you can do, spinning as fast as you can for as long as you can. Res for a couple minutes to recover and do it again. This is where a lot of your improvements in speed will come from.
The other training is intervals and it too will tax you to 100%. Search on here for "sprints intervals" and you'll find plenty of info on doing intervals.
So the bottom line is, in order to improve from where you are now, you need to ride HARDER and LONGER than you're currently doing; just not on the same day. Take out two of the medium rides you're doing, and replace them with one much more intense shorter ride and one much longer easier ride.