Yummy cow food?
Then, while you're eating all that yummy cow food, walk for an hour at lunch and another hour after work.
gag
Seriously, vegetarians *do* tend to not be overweight. I've heard that "true" vegans and many vegetarians actually are often hard-put to keep themselves *up* to a healthy weight.
Also, I have read somewhere that a lot cheap starchy "fat-free" foods tend not to help. Vegetables, some starchy foods, some meat maybe two or three times a week. Follow the new "food pyramid". Consult a nutritionist who is actually up on the latest research into the subject.
Walking may make you healthier and feel better (personally, I prefer biking because even gentle walking or jogging and certainly running jars my back too much), and is certainly not a Bad Thing, but it hardly burns off enough calories to count, especially if you're still gobbling America's Favorite Fries or whatever.