Even better, stop by one of the visitor's centers along the C&O and pick up a map &/or one of the little green guide books that includes the mileage of every campsite, culvert, aqueduct, bridge, boat ramp, etc, etc, etc. Then you can look at the scenery and still be able to figure out your mileage.
An excellent suggestion, and I will do that before I do the whole towpath next month.

In the meantime, I think I will get myself a bike computer; that would solve the problem regardless of where I ride -- 25 miles up, turn around, 25 miles back! (Assuming I lose my mind and decide to do 50 miles again; maybe I should simply hope I won't do it in 90-degree heat again!)

But, you did it. What doesn't kill you...
I was anticipating feeling really proud of myself for doing my first half-century; what I felt instead was stupid!! Maybe once the stupid feeling wears off -- assuming it does -- I'll feel proud of myself, but so far, it's not happening!!

Thanks for the pointers
You're welcome! This old Owl is learning new tricks; a pity some of them are learned the hard way, but I guess that means the lessons will stick, right?

A little of the massive amounts of crud I got off my bike:



The vultures anticipating my demise: