As part of a recent patch, mutators have been added. The game now allows you to change aspects of the match, and all of these matches still count. The fastest method is to crank the speed up and set the match to have a 1 goal limit. Do so with a no-bot 1v1 and all you have to do is boost into the ball, kick it in, and claim your match victory.
Doing this, it's possible to get about 3 match completions per minute. Conversely, if you still need to go for the 500 km trophy, crank the time to unlimited and just drive around.
Just an update on this trophy: When last I checked 11/11/2015, the number of matches that needed to be completed had increased to 191 due to new items. According to the v1.13 patch notes, this number should have been reduced back down to 150 matches
. (Thanks to DRedWingsDJY
for letting me know)