Go to multiplayer, monthly, scroll all the way right and select 90 minute spa.
You will be forced to purchase a very expensive car, the Lotus E21 for 2 million CR or 3,334 TK.
Once you have the car, you'll hopefully be thrown into a lobby with a short wait period until the next race. If there is more than 40 minutes left, exit the lobby and keep retrying until you get in a lobby that has a wait of less than 30 minutes. The best I could get into was an 18 minute wait.
Now once the race starts, your main focus should be on hitting 140 miles total driven before the 90 minute clock expires. People will mess with you and try to damage your car. Always know where others are by watching for blue dots on the mini map!
If you do get damaged, don't panic. Keep racing until you see the finish line. The pit is located to the right after the final turn. Just hug and follow the wall. You'll find it. Be sure you are driving about 12 mph or less as you enter to pit or else the game will try to force you back onto the track.
140 miles in 90 minutes may sound a little rough, but it's really not that bad. I was constantly damaged by others and had to pit several times over the course of the 90 minute period. You have a good bit of breathing room and I can tell you I am not a Forza Professional by any means!
I first hit 70 miles (half-way to the goal) with about 48 minutes and 50 seconds remaining.
I'm sure you can race better than me, so you will probably have 50+ minutes remaining.
At the absolute minimum, you should have at least 70 miles by the 45 minutes remaining.
I achieved 140 miles with exactly 10 minutes remaining and ultimately finished with 160 miles.
You must not quit at any point during the race! Even once you have hit the 140 miles, you must stay until the challenge pops!
Good luck, it's really not too bad. It just can be frustrating when people mess with you and try to spin you out.