I believe that the OP just exceeded his present fitness level. I'll use my wife as an example. When she was 55 she rode across the U.S.,averaging over 55 miles a day for 74 consecutive days on a fully loaded touring bike. We were both working at the time and did not have much margin for error. She was in top shape, and knows how to train and how to pace herself. Since then she has completed at least one multi- month tour each year totalling 25,000 miles. She was a pretty good triathlete, so she knows how to train, and set goals; she really knows how to handle a bike.
We have a little hill we come down, reaching 25 mph. If the wind is not too bad we see if we can keep the speed over 20 mph for the next mile on the flat. On today's ride she beat me by a good 20 yards maintaining 21 mph, then she dropped down to 17 mph for the next couple of miles. She was riding her 28 lbs. touring bike. I blame her "win" today on her age; she is only 65 years old.
I think that if you just go out and hammer without an objective or a plan it is pretty easy to exceed your fitness limits.
Last edited by Doug64; 08-16-20 at 10:17 PM.