This refers to the 'Instant Missions' which are separate from the main game. I recommend waiting until you can at least buy the 'Decimator' then once you have it upgrade it a bit.
With a decent bike press the

button and select 'Instant Missions', scroll along to 'Sprint' and press

to do the 'Subtroit Sprint'.
Now simply fly down to the finish line as fast as you can, shouldn't prove very difficult. The achievement unlocks immediately.