I have a short commute, about 4 miles each way, without much elevation change (going in, drop 140 ft then go up and down a bridge then climb 40 ft). The only way I've found to get any exercise is to push hard, try to cut my time down, race buses and lights. I can get my heart rate up that way, but only for 10 minutes. It is better than nothing, but I think it burns at best 300 calories a day. About a bagel's worth. But getting to work awake and ready to work, even if sometimes damp, and saving $200/month in parking or $120/month in bus fare, is nice. I save a bit of gas too, cars get atrocious mileage on such short trips. If your car is rated 20 mpg city, in a cold start 4 mile trip in cold weather, it might struggle to get 15 mpg.