I just started sprinting regularly over this past month. I do mine on a treadmill. I start out walking for about 5 minutes to warm up, and always have an incline of at least 2%. I then put it up to 7 for 1 min, then back to 4 for 1 min, then up to 7.5, then back to 4. I do this in .5 mph increments till I hit 10.5. By that point, it's been almost 20 minutes, and close to 2 miles. I am spent by then. I know the treadmill is not primal, but I like the idea that the treadmill forces me to maintain a certain speed. It's a little easier on the joints too.