I tried to read through the responses so if I missed someone saying this I apologize. I have a few things to contribute.
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When Nate says he did Leadville on 2 hour indoor training rides we need to consider two things. One is that Nate has a high FTP and from all appearances is a gifted endurance athlete. What works for Nate doesn’t necessarily work for everyone else. Two is that Leadville is darn near a gravel race. It is not a full on MTB race with technical climbing or descending.
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Mountain biking requires more core, circular pedal stroke, foot strength, arm strength, etc. compared to indoor, road, or gravel. Especially in a longer race format. I very often cramp in my adductors and feet at the end of MTB races toward the beginning of the season. I have to pedal a rounder stroke to get through technical terrain and descending puts more strain on my feet that pedaling.
My point is that MTB racing and riding requires a ton of other muscles to be strong. From your neck, to back, to arms, to legs, to even your feet and toes. Riding inside cannot prepare those muscles for what it takes to ride single track climbs or descents for a 4-6 hour race.
I suggest getting out more often to ensure your other muscles have the strength needed to perform in these long races. You can do more core, balance board, dumbbell rows, etc. to improve as well but getting out and doing 3 hour MTB rides is a great way to do it.