It depends....some sections I can easily maintain 23-26 mph for a stretch (solo).....other areas I feel like I'm gonna die pushing 21-22 mph after awhile. Really depends on the wind, road surface, and gradient (varies too much around here to find a really "flat" stretch for any sort of distance).

Like yesterday for example....went riding with a Cat 3 guy at work, and this one section we were hammering along between 24-28 mph for a few miles (we weren't drafting each other or anything during the ride), including some slight uphill and downhill gradients (but mostly flat), and had a GREAT road surface. If that road kept going like that, I felt like I could've gone for an hour at that rate.