On the MTB I think mostly about what I’m doing at any given moment TBH…
But on the Road bike, especially on long straight forwards familiar routes (eg when commuting) my mind can toddle off a bit on to other topics and the autopilot kicks in, which is great as you sort of make progress without really noticing any leg pain or tiredness…
Of course the second you actually notice that you’ve tuned out, reality sort of jumps back in and you have to deal with all those mundane concerns…
I seem to be a shade quicker when my brain switches into that mode, like 90% of the actual cycling becomes almost “autonomic” I guess the times I’ve been faster/braver on the MTB I’ve had a similar “not thinking too much” moment..
Not thinking too hard seems to benefit me…