Personally I've never had a radiant quest that sent me very far from the quest-giver. Also, those quests usually aren't timed, so I just do them at my leisure. My main concern with "no fast travel" will be settlements being attacked. They always seem to happen on the far side of the map, and often when I'm in the middle of clearing a location. I haven't tested how long the game really gives you, but settlement attacks are the one thing I always fast travel for.