F1 2021

The probably fastest way to boost this achievement is to play with a boosting partner. Select 3 laps on Austria with max AI difficulty. Once the race starts, open the menu and the AI drives automatically for you. You can even let the AI finish the race. This should bring you to 12-15 races per hour, so it still takes some time, but most of that can be spent doing other things.

League events are intended to be played in multiplayer, but can actually be played solo. When creating a league you have to set a time for the race to take place. Just choose a time in which you can comfortabely create the league. You have to select 5 tracks, but for this only the first track is important. The shortest option is Austria, if you select the shortest race length (3 laps). Optionally you can reduce required drivers to a minimum of 2 (so 1 AI driver). After creating the league just join the first event and wait for the timer to run down and the event should start with only you in the event.

If after doing all the other achievements you still need more track time for this achievement, you can start a time trial and stop the car after crossing the start/finish line. You have to keep the game focused though, as otherwise the game and with it the time will pause. If you still want to use your PC while idling the game, you can create a league like in the guide for the achievement League Racer, but set the race length to longest and the AI difficulty to lowest. When the race starts just open the menu, the AI drives for you and the time counts. Regularly check if the race is about to end, as you have to create a new league for another race.