Many treadmills have various workouts preloaded into them already. We will usually do a program called a sprint 8. It starts off with a 3 minute, 3.5 mph, 0 incline walk. Then for 8 sessions, a 30 second, 6 mph, 5 incline run, with 90 seconds in between runs. This takes about 20-25 minutes with a 3 minute cool down. We follow this up with various weight training for 30-45 minutes, then to finish a 1-2 mile run at 5-6 mph and 0 incline. There are so many programs you can come up with. You have to find what works for you. Another thing which is tons of fun are parachute runs. If you can get your hands on a parachute, strap that on your back and do some running in a parking lot or field. These will get you in shape.
Another good treadmill exercise for your legs is do a 3 min warm up, 3mph, at 0 incline. Every 2 min go up either a half mph and half incline. Work your self up in speed and incline as it is comfortable for you. Long legs prevail over short in any running or walking exercise. When you have reached your peak with the speed and incline, do it in reverse working back down to your starting point. I will do 30 min up and 30 min down. This will work your quads, calfs, hamstrings, and gluteus.