Running downhill produces a large eccentric force especially on the quads which can lead to a lot of soreness. You might want to try easing up on the downhills. Although some runners say a hard downhill run about once a month can help make the legs more fatigue resistant.
If it's normal for you to be unable to stand for the rest of the night following a run I'd say you need to ease up a bit on the speed or distance of your runs. Scaling applies to running the same way it does to lifting. Just because you can finish a run doesn't necessarily mean you should.
You're also doing a huge amount of volume. Are you sure you're eating enough total calories?
And with the high volume you're doing you will need to take 3-7 days off once in a while. Remember, you get stronger when you rest, not when you're working out.