A major factor with cycling is body weight. Those extra pounds make things VERY difficult. I always get a kick out of these guys who show up on their $5000 15-pound carbon wonderbikes ... and they're 25+ pounds overweight
My usual weekday ride is a flat 20-mile loop with no rest stops - a comfortable pace is around 17, when my legs feel good I do it an hour (20MPH average with several hard stop sign sprints mixed in, a decent workout for a 56-year old ). We usually do a 40-60 mile ride on the weekends with a couple of 5-10 minute rest stops.
1 MPH average speed increments in cycling are huge! And always remember Greg LeMond's famous quote
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And can somebody edit that Vimeo video a few posts back so it doesn't auto-play every time you click on this thread!!