The new treadmills at my gym won't come up to 15% quickly, making it difficult to do a hill interval session. They claim that it is a safety feature - more likely they use a really pissy motor on the incline to save money. So I've started doing the session by keeping it at 15%. So, start the belt at a slow walk - say 5k, and then hit the up button to 15%. Once it is up wait until a minute comes around and hit the speed, which comes up a lot quicker than the incline, to say 11k. You might have to release and repress the button at 10k. A minute at 11, then back to 5k for a trudging recovery for a minute, then back to 11. I find I can average only 8k, but I can make the efforts really violent, ie 5/11, or 12/4, 13/3, 14/2. At 5 and 11 I can count every other step on the right leg as "1", "2" etc, ie, right thump, one, right thump, two. There are 34 such steps in the minute (or 136 real steps), and if you count them like that the minute goes in pretty quickly. In fact, the recovery is worse than the effort. Sharp little workout - twenty minutes and you are a gasping mess, and the low speed means that it doesn't damage your knees, but it busts your lungs and works your legs. Don't think I'll do it more than once a week, though.