I racedXC durning COVID last year and have got 2 races completed this year.
Pretty sure they will do TT staggered start, like group of 3 to 5 every 1 to 2 minutes.
This really helps with congestion when you it the single track.
Warm up is important, typically there is a section to the single track and that is where you want the hole shot.
Once on the trail just be aware of who may want to pass they will let you know and just find a place where you can slow and make sure they tell you left or right depending on the lay of the trail.
Have fun.

It really depends on who is behind you, some guys will be nice and ask to pass and wait until you give them some space, some will just blast by you and practically push you out of the way.

I've done a few 24 hr mtb races (never solo - I'm not THAT stupid!). But in those the usual is for the faster rider to to call 'on your left' or 'on your right' if they're coming past.

But also it's their responsibility to not be a twat and try to get past when it's not possible. You don't have to crash out to give them space if there's no space available.

So basically common sense and mutual courtesy.

My experience is the really fast are fine and get past without fuss. And they say thanks. Its those racing for 87th place but think they're better than Nino Schurter who are the dicks.

Funniest thing I recall ? About 3am, Sleepless in the Saddle 24hr event one year. The laps are about 8 miles long or so.
I manage to get to the top of the one good singletrack decent on the course maybe 10 yards in front of one such dick. I'm a WAAAY better descender than climber (as I'm lazy and unfit). He's pissed off that a fat middle aged guy is in front of him. He thinks he's faster than me downhill just because he's fitter than me uphill. He tries to catch me up downhill, but can't, for about 200m... then 20m or so behind me there's a clatter, the sound of breaking bits of shrubbery, a thud and finally an 'euurrrgggghhh' as a big tree doesn't move out of his way. Dick. Actually a dick who trains too much but doesn't ride a bike enough.

Given the account you just gave (which you deemed "funny" despite the fact that someone crashed going downhill, in the dark), talk about projecting onto others your own faults

If memory serves, I always started MTB races way too fast! So a good warmup and use of common sense not to burn all your matches up the first climb :)

Envious of your in-person racing opportunity!

As others have pointed out, a good warm-up is always advised. That said, if you are doing this for fun it may not be imperative to get up front before the single track starts. It’s not your responsibility to slow down so others can pass. You just need to give some room when it’s there. On the flip side, be prepared to expend some energy when you pass. Bring a post race beer. That’s the biggest difference between off road and on.

I actually went and pre rode it yesterday, glad i did as there were enough technical bits, quick climbs etc that would have sucked riding totally blind during a race. There’s a few minutes of wide fire road that should spread people out a bit, but I’ve no intention of muscling my way to the front to get their first, as it’s truly just for fun