Lose a pound, run a mile two seconds faster. That's a good rule of thumb for distance racing. (Metric equivalent: ~3 seconds/kg/km.) Like any rule it's fun to extrapolate, to push it to its limits and discover the point at which it breaks down. Eventually too little muscle mass can't propel a person; in the other direction, too much of a load causes skeletal systems to fail. And there should be an age factor, maybe ~1% per year. (Alas, I am becoming more aware of this as I get older.)
But by and large, ~2 s/lb/mi is amazingly accurate. The marathon makes a handy test distance, since it fortuitously converts the relationship into an easy-to-remember "run your weight" goal. It's tough for a man to go 26.2 miles in less than ~1 minute per pound of net body weight. Women tend to have an additional handicap of ~20 minutes, due mainly to differences in the percentage of fat that the sexes naturally carry.
Which brings to mind one of the stranger fragments of conversation overheard in recent memory. At the Rockville Twilight 8k last month I was standing in the pre-race latrine line behind a buff pair of teenagers, male and female. "Have you thought about surgery?" he asked her. "Yeah," she replied, looking down at her breasts, "I think I could gain 20 seconds if I had them reduced."