Well it has nothing to do with boxing, but i do stair climbing (outdoors). About 20 mins from my house is a long, steep set of stairs, so i walk there, go up and down 10 times, then walk home. Sounds boring i know, but i don't mind it. I do it first thing in the morning as im dieting. Takes about an hour all up. Intensity is pretty much perfect for me. Harder than walking, but not too intense that i'll burn too much muscle.