I love riding on my local MUP as it's essentially endless, it's well engineered and it's maintained. In cool weather or early AM, I can do 20 MPH on it all day without issue. I live in a densely populated region but surprisingly the MUP is almost always sparsely used or even deserted. At 10am on a summer Saturday, forget it.

Walkers and cyclists use the same side so I'm very familiar with the issues you mention. I've found that if you use a bell people are very respectful and reactive. A bell is non threatening and people seem to like it more than "on your left". About one out of every five runners is completely dead to the world and nonreactive when I ring the bell / call on your left. You have no idea if they've heard you so you're forced to creep by giving lots of room. Still, I'll take that risk any day over the insanity of local drivers.