I ride one MUP as part of an occasional ride I take -- it just avoids a crappy stetch of divided 4way shopping center trash road. If there is almost nobody on it (cold/windy) I'll ride along at 18 or 20 mph. In that situation if I have to pass someone I can see them a long way ahead, and get an idea what they are doing. If they're a rider/skater/runner out for some exercise and appear to be holding a line I'll move the the far side of the path, giving them a good 8 feet of room at least, and probably just ride on by. I will slow down a bit and cover the brakes as I approach, but slowing to 5mph doesnt' seem needed.
If the path is busy, I generally am not topping 12 or so. When I have to pass someone in that situation, my speed and tactics still depend on whether they appear erratic or appear to have some general sense of the space around them. For kids, slow to a crawl and give as much space as possible. Same for bad dog owners with extendible leashes. Same for clueless women walking four or five abreast (sorry ladies, guys don't seem to do this) with no sense for the fact they are occupying the entire path
. But I usually avoid the MUP at busy times anyway. Unless I want to enjoy the scenery.