Running

Don't lead.

Simple.

Some are 'flagged', others use some identifiable paths etc. Others are free route choice. Some races have sections of each, e.g Boars Head (iirc) .

FRA website useful resource.

If the race is advertised as PM it means partially marked, but it doesn’t guarantee much.
Generally though, the shorter the race, the less nav required - people to follow etc.
You’re unlikely to get lost on most races but map and compass is pretty much always mandatory.
The longer a race, the more spread out the racers will be, making it easier to go off course. That said, following people is no guarantee either - been at plenty races where big groups have gone the wrong way.
For Ennerdale and Wasdale, I’m going to recce both routes - hopefully more than once each - because I know I’ll be at my physical limit so I won’t want the stress of uncertainty with the Nav side.

As for knowing where to go/best line to take etc, it varies from course to course. I gained a good few places on the Rydal Round (Fairfield Horseshoe), by taking the descent line with a big drop (3/4 metres of easy but vertical-ish downclimbing).