You have to take a full set of tyres - Then if it is too easy- you can resort to "bad tyres"
If you have the RA menu enabled you can play a few games
"Brake fade"
Roll a six sided die (Pinch it from your Cluedo set)
1 = Reduce Front Brake Bias by 1
2 = Reduce Front Brake Bias by 2
3 = Reduce Rear brake Bias by 1
4 = Reduce Rear Brake Bias by 2
5 = TCS Failure! Turn TCS to 0 (If you run no TCS - Give yourself a Beer)
6 = ABS Failure Turn ABS to 0 (If you run no ABS - Give yourself two beers)
This should slow you down a bit, but more importantly alter the race for you.
Other "Fun games" can be Gearbox trouble - You have lost the use the lowest gear.
Engine Oil temperature too high - Reduce your upchange revs by 500
Pit crew not ready - Some error with your tyres - you'll have to stay out an extra 1/2/3 laps
Bright spell - Sun has come out - alter the Brightness to its highest setting
Sun obscured - Reduce the brightness to it's lowest setting
The Random factor could make a race a bit more fun - memorable. BUT if you are in danger of actually losing - you can always go back to normal to get the important race win.
I am sure you can come up with some alternatives to break up a long enduro.
After every pitstop you can have the option to fix all the problems - Or that could be the result of a "stint roll"
Persoanlly I would dial down the tyres (I run no ABS or TCS so by my own set of rules I'll be racing with a car full of empty tinnies in the footwell.)