wow you guys have admirable routines!
I used to run like crazy in short bursts, like 7.0 mph alternating with 5.0 mph for 15 minutes but I read an article about how hard running can compress your spinal discs, and make you lose a half inch in height. since i'm vain, what i do now is just walk on the treadmill at a leisurely pace (like 3.0 mph) but at a very high incline, between 8 and 15%. This will give you really toned legs and butt and you'll still break a sweat.