The Meanest Link Canoe Route is an epic paddle that is "legally" allowed to be broken up into 4 separate trips so it could count as 4 training events.
https://algonquinoutfitters.com/map/meanest-link/
Another Canadian tradition is skiing so you could do the Canadian Ski Marathon or the Gatineau Loppet next winter. Gatineau is more than 50 km.
Thanks for all the suggestions. The Scottish 6-days would be wonderful but it conflicts with Canada 150. Challenge the Gats and Billygoat are great suggestions though I doubt I will be able to garner any more career points in the Billygoat standings. I am thinking about the Billygoat this year as well. Knee Knacker is definitely a no. My knees hurt just hearing the name. ;-)
I have to admit it is hard for me to do a race that doesn't have at least some sort of navigation in it.
I'm working on my own 50. Billygoats, that is. 2028 is the magic year. Not to mention staying healthy on all the years between, I've been pretty lucky so far (had foot surgery about two months before one of the mid-80s Billygoats, but these days I wouldn't bounce back so fast).