When I run roads in the winter, the muscle mass in my leg decreases noticeably, as does muscle tone. When I start running trails, the muscle mass and tone increases significantly. When I push up volume and intensity, particularly focusing on downhills, the muscle mass and tone increases and then stays steady. Running roads, my hiking w/backpack is very weak. Running trails moderately, it's OK. Running higher volume and a focus on intensity and downhills, it gets easier. I relate all those experiences to gains in strength. (Leg strength, obviously. No one, including me, is advocating that running increases upper body strength.) I feel best when I'm at 30ish miles a week, trying to hit 3-5,000 feet of vertical, doing strength work 2-3 days a week and then decreasing mileage and adding 2 hiking days a week as the season advances. Although I'm noticeably slower after that, and have to pick that back up in the wintertime with flat-and-fast running. All very interesting. Those are my observations, I appreciate the back and forth.