Three general comments:
1. I view "miles per day" more in the context of budgeting time for a trip, than a particular goal I am trying for on the trip. Two recent examples:
- I'm just finishing a ride this July from Abilene to Minot. Prior to the ride, I wanted a rough estimate of time to allow to budget my vacation time, return trip plans, potential adjustments, etc.
3. It has changed over time as I've gotten older. My average speed has gone down slightly, but at least as much is a preference for shorter days and taking an occasional rest days. In my 20s, I had more of an idea of cycling from early morning to late afternoon. In my 50s, I'd be happier stopping midday. This has been the biggest factor affecting miles per day. To give an example: My first trip across the US was 95 miles/day. Five years later I crossed Canada doing 85 miles/day. Four years later my second trip across US 75 miles/day and around Australia at 55 miles/day. Five years later crossing Russia at ~55 miles/day. Alaska to Argentina was ~30 miles/day with averages pulled down by time spent not on the bike e.g. a few months break here and there. Time I was actually riding would be closer to 50 miles/day.
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When you wrote "Abilene to Minot" my first thought was, BORING! Did you find yourself riding further on this trip that some of your other recent trips, just because you were riding prairie most of the time?