I can ride 50-60 mile rides with no extra preparation. These aren't unusual for me. But I only rarely do 90 or 100 miles, and it's a long day.
A couple of years ago I did one of the regional weekend event rides, 100 on Saturday. I expected that I'd only want to cruise 25 miles or so on Sunday, but went along with other local riders doing 50 miles, and finished it comfortably.
Just get through the first few miles on the second day! Those aren't easy...
Calories per mile
That mention of 40-50 calories per mile probably only applies to very hard efforts over the full ride.
My 100 mile ride last year had 6000 feet of climbing, with lots of smaller hills all day long. My power meter reported 2740 kilojoules. Kilojoules roughly
match up with calories. I often see around 30 calories per mile for my rides, just like this.