What's the single best way for most runners to improve their performance? Lose some weight. Here is a list of reasons to lose weight:
1. Your Max Vo2 will rise. Think of a car; if you tear out a bunch of extra junk from the trunk, but leave the engine alone, the all important horsepower/pounds ratio increases. And this ratio is the auto equivalent of your Max Vo2 reading. While Max Vo2 isn't as important to a 100 mile runner as it is to a miler, the idea is to have the same cardiovascular output but a smaller body to push up and down hills. If you weight 180 pounds now, and manage to lose 20 pounds, you'll use less energy and burn less calories at the same pace.
2. Less stress/pounding means less likely to sustain an injury. Running is a pounding exercise, and any lessening of this pounding is bound to help your knees, achilles, etc.
3. You are likely to live longer and suffer less non running related health issues. Ever seen a fat 100 year old? Seriously, just because you can run a 50 or 100 mile race doesn't meen you are healthy.
I was thinking of weight recently in the context of shoe weight. Finding the lightest shoes seems to be a common topic with a lot of runners (and I too prefer lighter shoes) but I find it odd that many people will agonize over an ounce here or there on their shoes, but neglect the dozen or so extra pounds they are carrying around on their bodies. Of course there are other reasons for choosing light shoes, as the lighter shoes often fit and feel better, and provide more flexibility. But, wearing the ultra light shoes also adds an element of injury risk (see 2 above) if you are carrying more than your ideal body weight. Just remember, a lot of the people you see wearing the lightest shoes are themselves light (Matt C, Tony K, etc). They would be great runners regardless of what shoes they wore.