..... it has begun to dawn on me over the past few years that actually, I may never repeat things I did when I was younger, things I always 'assumed' I would, like go back to India, scuba dive, Eurail with a backpack, climb Helvellyn, walk the SW Coastal Footpath.
I wonder how long ago it was when I last 'ran' properly? Or felt joint pains that then went away, didn't become chronic? Went on a 10 mile bike ride through the tracks in the forest? Camped? Hiked for miles?
I am also aware of things I may never do, like see South America or Japan. Or feel 100% confident in my body!
I know there may be many who will tell me of their ultramarathon from a standing start, at 60..
... but I can see how we turn into 'old' people, and at different ages. I work in health care, and I see 'old' at 55, and 'sprightly' at 75.
FTR, I'm 56 and yes, in a rut. But menopause destroyed my mojo (on an enforced break from HRT...) and manual handling in the NHS wrecked my spine and knees. 
I honestly thought that at 56 I'd be hiking up Alps but walking to the local shop is knackering these days!
No particular reason to post this, just my musings on how 'age' creeps up on us.