As Wintr17 said, you have to reach the large treasure chest at the end of the path, but rather than completing the crossword one, I'd suggest the jumble one - although there are more (80 Jumbles + 5 bonus), they are a LOT faster to complete, especially if you use a website for the words.
I used the site
http://www.wordplays.com/anagram-solver/ which gives all words rather than just the largest, if you start from the longest or the shortest and work your way through then an entire puzzle should take 2-3 minutes (including the bonus puzzle).
You can also quickly complete the bonus jumbles during the run, they take the same time time to complete... just as an example, I completed the last 5 puzzles plus the bonus to get both achievements in around 10 minutes!
The related achievement is A Long Walk (I don't know how to show the achievement link in the solution)