According to British middle/long distance coach Frank Horwill, Most coaches use the Stillman height/weight ratio table for distance runners. The average man is allocated 110 lbs (50kg) for the first 5 feet (1.524m) in height. Thereafter, he is allocated 5½ lbs (2.495 kg) for every additional inch (O.025m) in height.
Females are allocated 100 lbs (45kg) for the first 5 feet (1.524m) and 5lbs (2.268kg) for every inch thereafter.
According to Stillman, top middle distance runners should weigh in 12% lighter, while long distance runners should measure in at 15% less. (Peak Performance Online)
Clearly not everybody who runs can fit into Stillman's criteria, then again not many of us are good enough to challenge world records either. But if you are over and not following the best diet, losing a few pounds probably won't do anything but help your running.