If you are sad that the Indy Endurance finally died as of v6.3 (I haven't personally confirmed it yet but numerous reports stated as much), well, I just found a new one, which, depending on your tastes, may be better or worse than the Indy Endurance.
Enter Lamborghini Huarán Performante Exclusive Series 20.2: Endless Endurance at Monza Road Course.
Similar to the endless run at Leipzig short, go full manual and cut all the corners that you can (especially turn 1). The bot train starts around mile 30 or so. I drove safely at first (never exceeding 90 sec), so if you drive flat out from the beginning you might catch it earlier than I did.
The bot train, once it starts, is just like the bot train at Indy Endurance. These bots have speeds above a maxxed out Performante, but they are slow around corners... and that's where you can beat them. By the time I caught the train after mile 30, I was down to 1x seconds, but was able to add ~ 10s per lap and eventually filled up my time gauge all the way back up to 90.
The good thing about this endless endurance is that you don't need to keep your car pristine. I had a bad wreck at mile 55 (rear-ended a bot and did a 180), but thankfully I had enough time to catch up to the train exactly one lap later. I eventually rammed into the barrier at turn 8 at mile 75 (too aggressive when passing a bot) and that did me in. Otherwise, 100mi+ is not out of the question.
Managed to snap a shot at mile 47:
Car is totally wrecked (except for right side) at the end:
There are some similarities with the Indy Endurance. Both bot trains start appearing after a period of lull, and both the Performante and P1 GTR have tremendous grips, well beyond their stated values. In fact, I think Performante has slightly better cornering than the P1 GTR based on my tests at Le Man. The better-grip-than-indicated point applies to the FXX K as well, which itself has the endless endurance at Leipzig.
Is this a good farming run? Probably not, given the low fame/min compared to Le Man 10-lappers. But at least we get another endless endurance race to replace the one we just lost.
Now, let's see those 1000mi. runs from fellow racers