Comment by exabrial

I did actually! I bought a gravel bike and [trained for, competed in] 104mi Unbound this year. It has definitely had a positive effect on my mountain biking ability; I can climb faster/longer and my heart rate settles faster.

What's more hilarious is the reverse cross training of mtb->gravel. The Unbound course is notoriously unmanaged "gravel" (pile of sharp rocks) for certain sections. Most roadies are squeamish to hit rough terrain at speed, and the ability/ confidence to pass _a lot_ of riders helped me move up quickly in the front half of the race. Even the ability to cross wet/rocky water crossings without dismounting added up quickly in both energy spent and time.