Don't mountain bike miles count for more? They are usually much harder miles, aren't they?

At this point, I don't have any specific goals--I just ride. When I was a runner, I counted every mile religiously, and kept to my goals even if it wasn't healthy. Now I just try to keep balanced, hard/easy, and get 2-3 runs in per week as well. I think if I ever do set mileage goals, I will weight the miles according to the bike, terrain (hills v. flat), pace, and also give myself credit for running. I don't know quite how that would look, but if I go on a long, grueling mountain bike ride, it's going to count for more than a spin down the paved road. Maybe I'll have to include a weather factor, too--temperature and wind. I've learned I have to keep track of wear and tear on my body as much as mileage accrued. But then, I'm not as young as I used to be!