Which endurance race? The Seattle one where you win a Super Bee Hemi?
Use something fast with Hard Sports tires ( i think they only allow Standard and sports). If no car limits, use something in the same class as the AI are, add turbo or NA tuning (whatever car you're using) and get every sports tires. B-Spec it, put on Fast Pace or Steady (whichever is better in that current situation), put on Overtake and you'll be first in do-time. after your first place and about 100m ahead of 2nd car, leave overtake on and put pace on steady. You Can do this all while fast forwarding, just remember to turn your pace down to steady when reaching a corner and putting it on fast on straights (if you're not first yet). I personally don't care about a challenge in endurances I just want to get the race done in guaranteed first place and less time.