Recently I started intervals with an incline. I've always been doing intervals, but since I love my booty and would like to keep it the size it is or bigger, I added the incline, walking/running at an incline actually helps those booty muscles. Incline intervals are amazing. I love them with all my heart.
I was up to 45minutes running intervals prior to adding the inclines. I started off at 10minutes with a 1% incline, then 2-3days later I increased the incline to 1.5% and now I'm at a 2% I was able to go for 30minutes last night. I'm trying to get back to 45minutes at 2% and then increase it again to 2.5 and then 3% and keep this trend until I make it to 5% and then I'll decide if I'll go up some more.
I also do intervals on the elliptical machine. I'll start at a lower level and increase to a 3-4 level difference at whatever they have their interval minutes at.
Here is some more info about intervals and why they are so great for our bodies.