I've been slowly increasing my exercise since Christmas, and am now cycling about 45 miles a week. The weight still seems to be coming off, and I'm feeling a lot more toned too. I think it's a personal choice thing really, try it and see what works for you. If you used to exercise before anyway, you'll probably be OK. If you're new to exercising, I would suggest start with a little and see how it goes. I think you'd have to do an awful lot of exercise to make a lot of muscle for it to really interfere with your weight loss. If you're worried about the calories, you could always have an extra pack.