The goal is strait forward, but endurance races are boring grind. To make it simple choose the most easy Drivatars, start the race, pass all cars and now turn all assists on, now you need to keep gas pedal on (put something in right trigger) and press some button every 20 minutes (to prevent auto-pausing the game). The car will do the rest. Passing the cars this way could make a crash, so you need to keep watching during this (it will be every 20-30 minutes) or pass them yourself and keep it auto-going. Hope that helps)
EDIT. It is better to put 20% Drivatars, so your car won’t overlap bots. Some races have a turn where your car always crash, so you need to come back and help it to pass that turn. Or keep watching all the time and help the car to turn, so it is ok to put 40% Drivatars for more money.