I think there is quite a difference between doing a lot of miles in one day and walking an LDP. I can walk 20 miles in a day if I have to but I'd much rather not, and certainly wouldn't like to do it 2 days running. On the other hand I am quite happy doing 14 - 16 mile days, and can keep that up for 3 weeks, and you can do most LDPs like that.
I did the PW when over 60 with a fellow Granny. We are both reasonably fit, but didn't train that much, as such. We both reckon that we might not be quite as quick up hills as we were 40 years ago, but we have more staying power. Yes, some evenings we were a bit tired (the first night nearly dead might be a better way of putting it.) But we made it, and felt pretty proud of ourselves at the end. We didn't do any 20 mile days I don't think. We meant to do the Cheviot ridge in a oner but wimped out when we realised the advantages of spending 2 nights at Forest View.
OK we weren't carrying camping stuff, but we didn't do baggage transfer either. Carrying a lot of gear might have meant shorter days, but if you are camping and not depending on the next b&b you can make the day as short as you want. There isn't a time limit after all.
As has been said, some of it is mind over matter. Actually, I think I would have found the Pennine Way harder at 20 than I did at 60-odd. And not just because there weren't any slabs on the paths then.