a sprint that takes 10-20 seconds is pretty good. if you have a hill nearby, sprint up the hill...it's a lot of work, but much easier on the body than flat sprints. basically, from your starting point, you run all out for 10-20 seconds, or until you just can't go anymore (that may only be a couple hundred feet) then slowly walk back to your starting point. rest for a bit, then start again. how much rest you take depends on your goals, but i would aim for 1-2 minutes. do that 6-10 times.