If you just want to get this done quickly (or need to because time is running out), go to Rivals mode, then pick the Time Attack mode for your preferred car's class. Once there, scroll right to Top Gear West. Race one lap to set a time, then pause and select Finish Race. At the Post-Race screen, select restart (not Continue) and repeat until finished. You don't need to beat 50 rivals' times, just complete 50 events. The award will pop as soon as you complete your 50th event, no need to back out to check.
I used this method to finish up the last 25 I had left in a little less than an hour. I did a few laps for fun to set a decent time, then knocked out the rest.