As Highlander mentioned, get Season 6 complete then begin the grind. I used the play and charge cable to keep my wireless controller from turning off as I didn't have a wired one, and that worked just as well.
If you're fortunate enough to work at a desk with a dual-input monitor like me, then you can do this while you work! I bought the $5 VGA cable from monoprice.com and setup my Xbox behind the monitor (no need for sound). During lunch I started one of the endurance races, hired a driver and it was done by the time I got back (hour). The rest of the day I would switch to Xbox, setup a race, hire a driver then back to work.
Took me 2 weeks straight, so that's 8 hrs x 10 days = ~80 hours in addition to completing season 6. Don't worry about being online, our firewall blocked me. I just made sure I was online during the last race when the achievement popped, which was on the event completion screen (when you're given the cash) after I completed the final race.