Sarah,
Running is really great for slimming down and burning calories, but if you can't fit it in (or just can't bring yourself to do it--that's the way I started out ) then you can sub some high intensity floor aerobics that are the same length as the run. The first half of Maximum Intensity Cardio, the Hi/Lo from Step, Jump, and Pump mixed with the Hi/Lo from 10-10-10 and Circuit Max would all be acceptable substitutes. You could also do step or kickboxing, but if you step is your usual cardio you'd want to mix in the floor aerobics to crosstrain (to bump up the effectiveness of the rotation) and help protect your body from overuse injuries.
Hope this helps,
Mattea