I agree with Dianyla: long enough for a pony tail or really short. I've tried the mid-range, and it was alot of work to keep it looking decent with working out every day. When I was younger, I kept in long and in a pony tail for running. Now that I am a bicyclist, I wear it short. Couldn't stand the crunchy hair (from salt/sweat) after a ride. My daughter got her hair cut chin length, and it drove her nuts, always in her face. That wouldn't present a problem on the bike with a helmet, but would it bug you the rest of the day?

Also, consider your climate. If you live where it's warm year round, you might opt for a shorter doo. Otherwise, winter is coming, and the longer hair might keep your neck warmer.