OP, I would strongly advise lifting weights. Don't spend too much time or energy on it, but increasing your calf, quad, glute and lower back strength can help considerably with the impact at higher speeds.
As a general rule, squatting 1.6x your body weight would be plenty of strength in the quads, glutes, back. For the calves, holding that much weight (like at the top of a deadlift), and doing controlled heel raises for 3-5 reps would indicate enough strength as well.
More than likely you are pretty far away from those standards if you can't break 75 in the 400. Your 200 speed should be in the low 30.xx range and 400 in the high 64.xx range if you want to break 5:00 for the mile. A 5:25 mile tells me you can probably hit a 70.5 or thereabouts for the 400 currently.