Totally agree with this.
Running for a couple miles is endurance. Hitting the bag is just fucking brutal. I regularly do boxing workouts, and it's still grueling for me to do three or four three minute rounds.
I haven't had any problems with my wrists, but I used to have problems with my hands. I bought big 16 oz. gloves from Title and learned to wrap them and haven't had any problems since.
Learn how to hit it though, even if you watch a couple videos at most and get some tips. Just so you don't break your fuckin hand by punching it wrong.... its very easy to roll a wrist when you don't know what you're doing.
That being said, regardless of what these other dudes are saying, it will lead to some kind of hypertrophy in your shoulders, especially the front deltoid. You won't look like Ronnie Coleman, of course, but you won't look like a pussy either.