This trophy will come over the course of the game. When you play, you advance ten metres per second at the beginning but you'll go faster the further you get in one attempt. The trophy is cumulative so this does not need to be done in one attempt. You should reach this figure well before you're finished with the other trophies. To check your progress on this trophy, go to The Stash > My Profile and Distance Covered is the third statistic under "Totals". If you aren't sure, 50 kilometres is 50,000 metres.