How to Discover Your Best Distance

Ask two runners if they think a 5K or a half marathon is harder and you'll get two different answers.
Some runners can cruise at a moderate pace for miles, while others are better putting the pedal to the metal for a short period of time. When it comes to selecting your most successful race distance, you must consider physiological, psychological and environmental factors.
Genetics endow some runners with a host of fast-twitch muscle fibers and others with a greater percentage of slow-twitch fibers.
"From a genetic standpoint, we are stuck with what we got—that's why we have elites and the rest of us," says Philadelphia-based coach Cory Smith. "With that being said, it is possible to alter how our genes are expressed through training to maximize our own unique athletic ability."
While most runners know from experience which distances they have a knack for, physiological testing is also available. Many NCAA programs work with GO! Athletics and System Based Training in Philadelphia, where they analyze everything from blood lactate to heart rate to determine a runner's natural strengths and weaknesses. The main reason a high-level athlete will do this kind of testing is to help guide training and capitalize on their natural gifts.

Your brain also plays a big role in determining which distances might be most fitting. Mindset, attitude and focus should all be considered when choosing what races to concentrate your efforts on. "Mentally, I think that people who are able to focus longer would be more suited for the longest distances," Smith says. "Personally, I find the shorter distances to be more stimulating."

Carl Leivers, a USA Track and Field certified coach in Atlanta, says that mental engagement is paramount. Before steering his athletes toward certain distances, he always asks, "What do you have fun doing?" What is a joy for a gifted long-distance runner can be a slogfest for a sprinter.
"I think most runners intuitively know what their strengths and weaknesses are," Leiver says. "Do you love going out and ripping into a set of repeat 400s, but you're bored to tears by a tempo run? Then I would say you're going to be better suited for shorter races like 5Ks and 10Ks. Or do you love to rack up the miles on your long run, but doing speed work is like pulling teeth? Then I say, don't fight yourself and stick to the longer distances."

http://www.active.com/running/articles/how-to-discover-your-best-distance