The point of aerobic training is just that, aerobic, or getting your HR up and keeping it there over a period of time. Aerobics can work certain muscles differently and that's where you need to make sure you rotate your methods or cross train. You will not "work" your muscles doing aerobics in the way you do when you weight train, so make sure not to confuse that. While it's true that walking on an incline will work your backside more (hams and glutes), you will get your HR up (depending on the incline), probably as much or higher than if you run on no incline (which I don't think you should do....you should always run on a slight incline because no incline is a little like running down hill IMO).
So, my point is, pay attention to your HR or Perceived Exercertion and pick your workouts based on that. Personally, I prefer to walk on an incline. It's just more comfortable to me and I can get my HR way up pretty easily. In fact, it's a nice workout if you do a lot of step which can emphasize the quads.
Carolyn
