Less than a week ago I typed this in a different thread:
.You have to train your body to handle multi-hour rides. I did a bike tour last year in April with an old friend and former co-worker, he is a year or two older than me. He used to be in really good shape, but he got ready for that tour with exercise that never lasted over an hour. Thus, his body never ran out of the stored sugars in his muscles and blood system during his training, thus he did not train his other organs to start generating the fuel his muscles needed after a couple hours. Thus, every day he was wasted after a couple hours. I was trying to get ready for a brevet, so I was doing 3, 4, 5, and 6 hour rides to get ready.
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Some people can get on a bike and do a five or six your ride without any training. I am not one of those and the friend that I did that tour with is not one either. Maybe you are, I am not saying that you are not. But, a long ride that you have not tried to get ready for can be pretty rough. Especially if there are strong headwinds or other inclement weather.
I did my first brevet at age 65, am 70, so do not let your age stop you, I see a lot of distance riders that are older than you.
Some of it is physical and some of it is mental. I have had some very miserable long days on a bike, sometimes you almost have to force yourself to smile. Photo below was on a 14 hour day. This was not a brevet, it was a bike tour where I was camping every night, and the last half of that day was a very tough day.
