If you want to do this achievement fast! You can do it over night! It might take a few nights to do but it's worth it as you don't have to do anything!
Iv gone from level 33 to 39 over night so another night should do it for me!
You can do it on a MX bike but I chose to do it on an ATV as I thought you'd be more sturdy not fall off as much! but the choice is up to you!
All I did was go on free ride and Sellotape
to the right and
to the left to all your rider will do is big doughnuts!
Leave it all night and in the morning the achievement will be yours!
You will average 5 miles every 15mins but the good thing is at the end of every 15mins you get (XP)
I got 500 points for the 5 miles covered and an extra 20% online bonus! So this will all add up to your level 50 xp achievement.