High Intensity Interval Training (HIIT) is a proven way to develop your conditioning. They're also excellent for fat-burning. And one of the best exercises for doing both is hill sprints.
Now, for those of you who think sprinting up a hill is too much for you, let's use our own definition of "sprint" here. While it's normally considered to be running at (or near) full speed, we'll consider it to be moving at (or near) your full speed. So a "sprint" might be a fast walk for some people; that's OK. The important part is the intensity, which must, obviously, be high. So moving at (or near) full speed uphill for a short time constitutes a hill sprint. See? I knew you could do it!