I've discovered that if you're willing to push your mountain bike up hills that you can't ride up, you get great upper body (more like full body) exercise. Suddenly you have an excellent crosstraining solution. Certainly no shame in that. And the best part is you get to ride the bike down. Wheee! Especially on Mt. Diablo's fireroads that appear to be covered with a fine ball bearing gravel which I've slipped on more that once and have since bought cleats for this is very much a feature.
If you are willing to mix the two (walking and riding) you will find yourself going places that you couldn't of before. Just remember to bring plenty of food and water as you can go further than you would of anticipated.