I planned on doing a 2 mile warm up then the tabata workout I read about in trail running, and know about from crossfit, but the 2 miler turned into a little more than a warm up and the tabata kicked my butt!!

2 miles in vibrams, 7:07, 6:49

the tabata workout consists of 20 seconds of sprinting 10 seconds rest, repeated for 4 minutes, a total of 8 20 second sprints. Try it, it's tough. real tough. But according to a study which I can't site off the top of my head it produces tremendous results in a short amount of time (the study was 6 weeks), increasing both aerobic and anerobic capacity.