For me I squat 225lbsx15 for 6 sets twice a week and I'm deliberately not increasing the weight beyond this point, I weigh 175lbs (6ft.2 and swimmer build so more muscle than average runner). I've noticed a big reduction of aches/pains in my legs since working up to this especially the knee, when I first started I could only manage 150lbsx8 in one set.. I feel like if you can't squat your own bodyweight for at least 20 reps then you shouldn't be running on those legs.