I would worry about your legs if I we're you, in my opinion give or take 2 miles a day is not far enough, even with loads of hill,stair and shuttle sprints mixed in you could probably up it by atleast a mile.

It depends on how your hitting the bags, if you not hitting them at 100% and going for speed and work rate then your muscles will gain muscular endurance, if you do them at maximum power (you've still got to keep the work rate up mind, you shouldn't be breathing much at all inbetween combinations until it hurts to much... pisses me off in the gym when I see lads loading up a 1 two then dancing around the bag for a few seconds till they catch their breath) you'll still gain alot of endurance but it will also build power.

Are you just wanting to add more muscle to your frame or are you actualy concerned with how you perform? I'm assuming your after size? why?