Ha, please don't shut up! All in good fun!!
The easiest logistically would be something like Waterloo Pinckney w/ DTE mixed in. Maybe not Syd's distance but up there? Todd would not have to solo support in this location, could probably rally a few other folks for portions.
WP=34 miles, does not use much of Poto
DTE=+20 for 54
Add Poto = 10 ish more? Certainly plenty of options to pad here and there, with various other trails in W/P, that would get to some really high numbers.
If travel is an option, there are other options more epic, eg 42 miles of Picture Rocks in one or more directions, or any significant section of NCT (esp. where Mark VT has had races). (Is that near where the infamous women's camping retreat drunk Cooper texting was?)
I still want to ride that High Country Pathway that's probably flat but is 80, if you just want distance. Only 4600' of climb in 82 miles so not great if you need hills. https://www.mtbproject.com/trail/7005013/high-coun...
If you want to go out of state, that brings up other choices as well, eg Ice Age Trail has a cold ultra on it in the scenic Kettle Moraine area. https://dnr.wi.gov/topic/parks/name/iceagetrail/
Frozen Otter I think.