Sooner or later, we all cut back as we get older. I'm sure when you finish the Half next Saturday, you'll admit you didn't feel like doing twice as much. Good decision.
I did my last running Marathon in 1991 when I was 50; my last 50km+ Loppet in Ottawa when I was 53. So you've got me beat by a long shot! Congrats!
only reason is the freakin cold and windchill of -15 at the start.
....and, maybe, the glide?
Yep 3-4 hours in that cold will be very uncomfortable, and the slowglide.
Hope you don't have to double-pole DOWN the hills too!