The Forza 5 Affinity levels take too long to get to level 25 if you play multiplayer it should take the most 10 laps to complete 1 affinity level.
It may take a while but at least it’s a level playing field. It seems to be based entirely on xp which is the same whether you are gold in a career race or come in 2 minutes behind the 15th drivatar. In multiplayer, if the race ends while you’re just halfway through, you get xp for every mile you completed.