The crunches will have a positive effect on your abdominal muscle group, but they are just like ever other muscle in the body...they can be overtrained. I recommend a total body workout alternating days with aerobic exercise. If you are temporarily on the sideline and your MD hasn't cleared you for weight training, swimming is another good aerobic exercise, which burns fat. Stretching exercises (like yoga) will increase your range of motion, limiting the chances of an injury in daily life or other types of exercise. I highly recommend stretching because it will improve your posture and range of motion. Be careful though, don't do more than you can handle, as injuries can occur.
Training all of your muscles will not only increase your resting metabolism but will also provide side benefits that aren't talked about as much, i.e. better posture, increased range of motion, etc. Your body can get used to a certain exercise and won't get any additional benefits. In effect, you are just maintaining a level of exercise/fitness. Same goes with weight training.
I also agree with another poster on the time issue. I didn't read the article, but I do know that in order to burn fat, a low intensity aerobic exercise (walking, swimming, Tai Chi, etc.) needs to last for at least 45 minutes to an hour in order for your body to move from burning glucose (comes from carbs we eat) to burning fat. I don't really know about the time issue...I think it would depend on your schedule and how soon you sleep after you exercise. To use myself as an example, I can't exercise an hour or 2 before I plan to sleep because of the endorphin rush I get after I exercise. My adrenaline is too high and I can't get calmed down enough.
An option for you could be to do another 30 minute session in the morning or at another time of the day if the 30 minutes at 8pm works for you. As long as your foot is better and feels good, I would recommend that. If you have hit a wall, so to speak, on your walking, swimming is an excellent aerobic exercise, like I stated before. Tai Chi is also something I personally recommend because of the fat burning benefits of an hour practice.
There is no such thing as spot reduction in any place on the body...it's a myth and doesn't exist. Just a little side-note.
Good luck!
Chris Gillette
2nd year M.S. student in Exercise Science (Sport Studies)