I have been messing around with very unstructured my version of fartleks recently. Rather than a steady pace, I have been combining sprinting, then some slow jogging, than a few step speed up, more sprinting etc etc. I find it tires me out a lot more than a regular 25 min fixed pace 3 mile run. Don't know if it does anything too much for my overall fitness or stamina. But like I said, very unstructured, and I might be doing it wrong/doing harm rather than good. The other thing I have been doing is speeding up as I run. Run the first mile at 7:30 min pace. The 2nd at 7:00 pace, and then the third at 6:30 pace. I find this more challenging than the reverse, where I bust my gut for the first mile (best I have ever done is 5:58), and then slow down for the remaining part of the run.