Is it possible to have more than 1 driver in a normal race? I have 3 drivers but since I can only use 1 at a time their is no reason to have the others.
Some of the later races you can take 2 drivers and they change during when you have a pit in... but they cant race at the same time in different cars for instance.
Easiest way to train endurance is to run him at Deep Forest in the 80's Japanese Cup with a Minolta. Set the speed to about 186 MPH as top and maximize Downforce. With this he'll just fly and level up very fast.
I think they get tired faster when the aggression is high for a long time. as long as you give him a car that is not too much work to drive and get in front of the rest of the pack.