Summarize the following:
Striding uphill, against the force of gravity, will force your legs and butt to work harder. Walking on an incline can also be easier on your knees.  After warming up, increase the incline on the treadmill every few minutes until you reach a 10% incline.  Your legs, butt, and lungs should feel challenged, but not exhausted. If you are unable to talk, lower the incline. If you have enough breath to sing, raise the incline higher. Steady cardio can actually lead to muscle loss, which is not what you want when you're trying to tone and shape your butt. Interval training means you alternate between periods of sprinting and walking, but it can be adapted for just walking, too.  Try applying intervals to inclines. Set the incline to 8% and hold on to the handles, digging your heels into the treadmill and lifting your knees. It should feel like you're dragging yourself through the mud. After one or two minutes, go back to a 1% incline and rest for a minute. To practice intervals without sprints, walk at a slow pace for 90 seconds. Then increase the speed to a quick walk, but not a run or jog, and do that for 30-60 seconds. Then go back to your slower pace for another 90 seconds, and so on. Pay close attention to what you're doing so you don't fall and walk at a slower pace than you're used to. Walking backwards expends more calories and makes your glutes and hamstrings work harder than walking forwards.  Because walking backwards on a treadmill is risky, make sure you are not overtired when you try it. Start with the treadmill on the slowest setting, then increase speed slowly once you feel comfortable. Set your treadmill on an incline for an additional challenge, or alternate between walking forwards and backwards to keep your workout interesting.

Summary:
Set the treadmill on an incline. Try interval training to burn more calories without reducing muscle mass. Walk backwards on the treadmill to engage your glutes.