First of all, you don’t need to run on extremely steep mountains when training for outdoor mountain running. If you do run up very steep hills, the only thing you’ll be doing is climbing. The ideal mountain running route should have a trail about a quarter-mile long with a moderate slope—not too steep—so that it can train the pushing power of your legs without over-tensing your muscles.
Secondly, try to focus on uphill running. This is because when you run downhill, the impact on your feet, ankles, knees, and legs increases (about 4 to 5 times your body weight). When you run uphill, however, the impact is much less, which is better for strengthening your thigh muscles and aiding in the stretching of your Achilles tendons. When running downhill, reduce your stride length.