I did this in Practice mode. You can use any car you want on any track. I used Ferrari F2004 on Nurburgring Nordschleife Endurance track.
As the track lenght is 25.4 km, achievement unlocked a little before the end of the 4th lap. Cutting laps is allowed, so knock yourself out. As you as you drive 100+ km in one session, you will unlock this achievement. Drive safely and good luck.