I'm following the progress of a 2:30 marathoner. She reports recently that she just ran for 4 hours between 5:10-4:55 per km. So that's 8 minute mile pace for a 30 miler. Is that super-slow? No. It's designed to teach her body to burn fat, as others have mentioned. Would it be a waste of time to run slower for her? Ask her coach, I suspect it would be a waste. Her body is in the fat-burning zone as fast as about 4:30 per km so I wonder why she goes that slow (5:10-4:55), maybe to make sure that she's not over-training?.
She's better than most of us. For the hypothetical 3 hour marathoner this would translate to about 9 minute mile running. Pretty slow. By the way she almost never runs faster than 10 mile race pace. This is pretty extreme, she herself has said it's almost like a different sport because it doesn't hurt, but hey, she's going to the Olympics this year and seems to have found her event.