I grew up doing the short and fat. It is mostly gravelish type roads/atv trail and double track ski trail on the birkie trail. There is plenty of climbing on the route. Lots of punchy fast roll down and fast roll up rollers. A couple longer climbs.

Relatively speaking, it is a pretty tame mtb race, half way between a real mtb race and a gravel race.

https://www.youtube.com/watch?v=NlOt-9TqxDQ

https://www.youtube.com/watch?v=ZCWhOfmgw1E

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been some years since i have done it.... but amazing event. 2+hrs of fast speed mtb racing on primarily nordic ski trails

You'll need a small gear for some steep climbs, and a big gear if you want to cruise the downhill fire roads.
I did it a few times 20 years ago; had triple front chainrings and used all my gears.

Hilly gravel riding is good training because the course is not too technical. Matter of fact, you could probably do it on a gravel bike with big tires if it's not too muddy (but it was always muddy).

BTW... did it the same year Greg LeMond did it after he retired. That was kinda cool, but not that big of a deal.