Losing Tummy?

I suffered a debilitating injury to my ankle and had to have surgeries to replace my Tibia, Fibula and Talocrural Joint. While recuperating, I packed on many pounds. I need to lose my belly and get back to healthy shape. I cannot run or jog, as the pain in my shins when I do is excruciating. I know that running is the absolute best cardiovascular exercise I can do. But since I cannot run, I have been walking 2-3 Miles a day, Jogging for short periods during the walk, as long as I can take the pain. If I continue this, with enough Sit ups and Push Ups, will I be able to build enough muscle tissue to burn the fat on my stomach? Any other ideas for good exercises that are low impact on my joints that I can do to lose my stomach?