Stop training for distance.
If you're like most runners, you train very differently for half marathons than you do for 5Ks, using longer distances for long races and track work for the short stuff. Today's elite runners don't train that way anymore. "We train all our athletes to have well-rounded fitness," says Brad Hudson, a Boulder-based coach and co-author of 'Run Faster from the 5K to the Marathon.' Hudson has all of them, milers and marathoners alike, do a similar mix of long runs for endurance, lactate-threshold runs (at 10K pace for most runners) for strength, and intervals for speed. With this combination as a base for your running fitness, you won't be limited by lack of speed in a longer race or fatigue in a shorter one.