What do we call the athlete who is in first place , leading the marathon or running in front of the other runners in a race?
Is leader an appropriate word to describe this runner?
'Leader' is a perfectly acceptable word to use in UK English for the front-runner of a race.
I don't agree with the use of front-runner here at all - at least for AE purposes. It doesn't sound right and wouldn't be used by an announcer of a race.
In marathons or races we hear the following words: