According to Google Earth, the walk from the MLL to the F rotunda is about ~2,500 feet, or half a mile. It's another 800 feet to the end of the F concourse. That's not an obscene distance, but it's not nothing.
It would be one thing if the MLL were right in the heart of T3 next to a UA Club, but my point is that it might just be out of the way enough for most UA fliers that many of them may not bother.