At the minute I'm training for the great north run, so I'm more into my distance running outside, but when I'm trying for weightloss and improved stamina on the treadmill, I tend to do HIIT, and I've been recommended but never tried Tabata training.
Basically with HIIT it's between 15 and 20 minutes, and you do around a minute of jogging at around 65% of your fastest speed, then a minute of sprinting at around 95% of your fastest speed. Give it a bit of research and I'd suggest starting with something like 45 seconds jogging and 30 seconds sprinting, and work your way up.
I'm yet to fully research tabata, but to be honest it sounds like absolute hell.
edit: I should also point out HIIT stands for High Intensity Interval Training, which is what everyone above has also suggested