First - You might want to consider getting a gait analysis from a reputable running shoe store, after which they can fit you with the appropriate shoe. Second - Unless you're bulked like Hulk Hogan , muscular legs will be more helpful than detrimental. I've been going through PT for the last few weeks for a nagging knee problem. I've definitely increased the muscle tone in my legs, and not only has my running improved, but I am less likely to end up with knee, hip, etc., problems specifically because I've built up the muscles that protect and support those areas. Strength training for both the upper and lower body is important for runners, which is why your physical therapist put you through those exercises in the first place. However, if you wish to decrease muscle strength and tone, simply quit doing the exercises. Over time things will go back to the way they were. I honestly don't mean for this to sound snotty, and I hope it doesn't. Best of luck to you ~