I’ve done a Metric Century on the road on a 36er with 125mm cranks and 50 miles at a 12 Hour Mountain Bike race on a 24" with 150mm cranks a few years go:
The 50 miles of MUni on hilly, single-track with some technical sections was definitely much more difficult than the Metric Century. I got to the point on my last lap that it was very difficult to free mount and my legs were simply on fire! After I finished I had a difficult time walking the next couple of days! Phew!