Endless Skater

I did this on the school level. Make sure to focus only on grinds. Its just barely possible, I had 5 seconds left when it unlocked. If you bail you might as well restart unless you feel you're really close to 500. If your lucky you get a run with long rails and curbs (which goes up, levels then down). Keep at it. Good Luck!

At the end it say best grind distance.

Edit: Should unlock in endless but if you bail the run restarts. As an estimate on time it will take 2-3 minutes to get the achievement if you focus on grinds only. Key is to memorize the level.