Do it. Maybe with support though.
I did Relentless 24 last year. Initially didn’t want to do it at all. Then grudgingly agreed to do it as a pair. Mate then bust his ankle, so ended up doing it solo.
First ever mountain bike race. Had never ridden further than 30km/longer than 4 hours. Had no time to train (and I’m not an XC rider at all).
Managed 18 laps, came 6th or something in my category, 10 minutes sleep (being a student used to doing all-nighters may have helped me with this).
However, the weather was perfect (cold enough for the duration to keep me cool, warm enough to not be freezing), no rain, and I had a great support team.
These two probably saved me.
I really enjoyed it, and recommend all my biking mates to do one (even if as part of a team).
Go for it!