fitness wise both the yorkshire and national 3 peaks are pretty easy unless you are below average fitness.
It’s very enjoyable though. You will find it challenging, but not so much that you can’t appreciate the scenery etc.
Avoid doing it at the weekend if you want to set a decent time and concentrate on getting up and down rather than dodging families with their dogs.
they will take their toll on your feet/legs if you don’t do a reasonable amount of walking.
as others have noted, get some miles under your belt on the hills with your boots on, every hour in them will be worth it when you do the 3 peaks.
I would rather do the 3 peaks with average fitness and great feet than great fitness and average feet. It doesn’t take much to get your feet ready tbh. You could even walk to work in your boots.