I am so hearing you on this point. I don't feel any different, but the person who looks back out of the mirror these days is an old man. I wrnt to renew my driver's license the other day and had to get a new photo. Oh, my gawd. The camera don't lie any more than Shakira's hips do!
On the other hand, we had a lovely autumn day yesterday with crisp temperatures and crisper maple leaves on the lawn. Tidying up and taking care of my bird feeders well crunching around fragrant red leaves is as delightful today as at any time in my life.
And so will your big walk be, I'm sure! So maybe you might have to rest a bit more from time to time than the last time you did the walk.
But you're gonna do it and love it. And that's what counts. :-)