A study with 40 healthy men examined four training methods: easy, steady runs; tempo runs; intervals of 15 seconds fast (95 percent of max heart rate/15 seconds rest); and intervals of 4 x 4 minutes at 95 percent max heart rate, with rests of three minutes easy. Training was done 3 times a week for eight weeks, and all runners consumed the same total oxygen in their training. Meaning, all groups did the same total amount of work The group performing the four-minute intervals won by a small amount over the group performing the 15-second intervals, according to improvement in VO2 max (increased heart stroke volume). The first and second groups fell behind. The study proves the theory that if you want to run faster, longer, get thee to the track for some long repeats! Try four reps of 1200 meters or six reps of 800 meters, and make sure to rest between intervals with a few minutes of easy running between reps. If you're new to speed work, build up to this amount, and make sure you have a solid aerobic base before attempting this work. There are numerous training plans and programs out there to guide you on how to introduce speed work into your running program. I'm partial to the structure advised by Jack Daniels, Ph.D. but please do your own homework to find what works for you.