My own experience of really fatigued legs was trying to get off Wheeler Peak some years ago when a thunderstorm suddenly erupted. Running along the ridge and then the steep descent to the lake had my legs trembling and so wobbly, I thought they would just collapse beneath me. Later after reviewing the entire hike, I realized I had probably become dehydrated and hadn't been putting fuel into my system either. Later that same summer I let myself become dehydrated once again while peakbagging in the Whites of NH. This time I made matters worse and was given a choice of hospital or 3 days at home drinking about a hundred gallons of cranberry juice and water. Fueling frequently and staying hydrated seems to have solved the fatigued legs issue. I've also taken to carrying poles, which I sometimes use. As Jim mentioned, they do help distribute the load. Some type of crosstraining or certain exercises to balance out the muscles might also help.