I'd like to add that hiking speed is just one part of the equation.

Miles traveled per day can be important, insofar as if you do too few of them, the weight of food you have to carry on your back can be excessive for some stretches.

But total miles traveled per day is based on average hiking speed AND average number of hours you spend walking.

Long distance hikers, particularly older ones, don't necessarily hike super fast. Some do, but just being efficient in camp and thus spending more of your time on trail can make a big difference. Particularly in late spring and early summer when there are so very many hours of daylight.

In that context, being in good enough physical condition such that you don't need a lot of long breaks and can just happily "keep going" can result in a lot of miles hiked per day without moving terribly fast or, hopefully, feeling wiped out at the end of the day.

I'm not saying that this approach is definitely right for you, but ... maybe?