It's great to have memories you can relive over and over again. Then again, you must be able to separate memories from reality. Two years ago, when I rode Land's End to John O'Groats (SW England to NE Scotland), I pompously wrote something to the effect that "while I can and have ridden multiple century days, there was no reason to cross Great Britain so quickly." 40 or 50 miles per day is soooo easy (compared to 90-100).
That line wasn't a fabrication. When I rode from San Francisco to Portsmouth, NH, we did half a dozen century-days back-to-back. Ah, memories! Here I am, three weeks from doing the Blue Ridge Parkway, with multiple 80+ mile days, and I'm suffering when doing 60 miles. My Plan A had been to work up to 80 miles by doing a long ride once a week starting in June, and by doing 80 each week in August. This heat derailed Plan A. I'll get a few more 60 mile rides in before we go.
Don't get me wrong, my legs are quite strong and can go the distance. It's all the other body parts that will complain. When touring, the average time is 10-12 mph, taking into account stopping to eat, take pictures, find a tree. So, even if your speed is 15-17 mph, the rest of the time is off the bike. Thus, I'm looking at 7-8 hours on the road. All of my joints will ache, my neck will be stiff, my new shoes are light but get uncomfortable after 3 hours. The lower back gets cranky after 4 hours (sometimes 2 hours).
I can still remember riding 78 miles, getting off the bike, and exclaiming "today was a short day." Those days are but memories.