Personally, I think a 20 mpw drop may feel pretty significant when coming from that high. I'd even feel a lot off a 10 mpw drop but I definitely think 20 is reasonable considering where you are in your training. It sounds like you're on a good path, just have to get all the pieces in place and I have a feeling you'll be fine on race day. I would probably speed up the goal pace, though. You do want to go at least 6:10-6:15 pace, if not faster, don't you?