"Bicycle widows" is the term for spouses of cyclists who disappear at 7 am on weekends and don't reappear until after 1 pm, at which point they're stinky and exhausted, so they're useless until about 3 pm.
The thing with casual cyclists of a certain age (my club's demographic) is that they like to eat long lunches. So you might be making a respectable 16 mph for hours at a time and then a trip to a diner tanks your average mph. Club riders tend to go for distance, not speed. On a good day those guys would average 11+ mph door to door. Most days was closer to 10.
If you're prepping for a century, I believe the protocol we used for new cyclists was to ride more than 75% of the target distance on the previous Saturday and more than 65% miles on Sunday. And a lot of that is just ensuring your body can handle the amount of friction you're going to deal with in 100 miles. If it was just the serious members, it was more like 80/70. You're likely going to go slower on day 2, so however long it takes you to do 80 miles, it takes about that long to do 70 the next day.
I would not suggest anyone try to go much more than 100 miles (my longest day was 112, and my dad went out after to round up to 115). After I went to college my dad got into 24 hour challenges, which seemed properly nuts to me, and it turns out actually is. Ultra-endurance athletes can damage their heart, leading to atrial fibrillation, which he got. He's been living post-stroke for about 8 years now. He was volunteering some place and was fortunate the guy standing behind him when he went down was a retired EMT.
