Flying a WW2 era fighter was a pretty demanding job. Some larger airplanes did have simple autopilots back then but I don't know of any fighters that did. The engine's fuel mixture also had to be carefully monitored and adjusted to get the best possible range. Navigation was also a constant challenge so that would have helped to keep a pilot busy.

The Zero actually provides a good example of these challenges. During the fight for Guadalcanal IJN fighters had to make long flights from Rabaul to try to hit Henderson Field. The challenge for those pilots wasn't boredom, it was exhaustion. That was part of the reason that the Guadalcanal campaign did such a good job of chewing up Japanese airpower. You had American pilots who could have taken off just minutes ago flying against Japanese pilots who'd been worn out from a multi-hour flight.