The best approach depends on what the problem is. If that leg/foot is fully functional and just a little weaker (maybe due to periods when you weren't able to use it much, which can cause muscle atrophy), then you'll probably need to do a little extra strength training for that leg individually, so you can use the proper amount of weight. The most efficient approach for muscle building would be to aim for 6-9 reps with a weight that gets you to the point of muscle fatigue by the final repetition. That would probably mean using gym equipment, or using some equipment (weights, resistance bands, etc) to add resistance to exercises like lunges and squats). Doing more reps with less resistance can help with endurance, but not so much with building up the strength in that leg.
For hip flexor issues, here's a blog with some suggestions for both strengthening and stretching those muscles:
Hope this helps.
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(Lennon & McCartney, "Blackbird")
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