Hmm... It might be possible. In the summer I occasionally take a long biking run which is about 30-35 miles, there and back. I want to say it takes me 5 or 6 hours (but we stop and have lunch too).
So, doubling that, 60 to 70 miles would take (me) 10 to 12 hours... with some meals in there. Maybe 13 to 15 hours for 80 miles?
EDIT: I think that a less-than-fit person could do it. You could always pedal a little slower at times. Biking doesn't exert as much as, say running does.