About once a year the company I work for will have a meeting at a place that's about 40 miles away, I'll ride there and back, admittedly with a 2 hour meeting in between, but I'm back home by 1 PM and it has never seemed very hard. My daily ride is 11 miles one way, 5 days a week, during normal commuting even with errands I have never topped 28 miles a day, but doing 80 miles a day wasn't really any trouble.

I guess it depends how you ride. I'm a slow and steady guy, my average on commutes is only about 16.2 MPH, I don't push much unless I'm trying to keep up with traffic through narrow areas or construction zones or something, so generally I feel like I could just keep cranking all day.