The Marathon Challenge will automatically begin within the latest city you've unlocked with normal progression (not with travel vouchers).
The challenge covers the latest 10 levels you've unlocked. If you have played up to level 22 in Budapest, the event will cover levels 22-31 in Budapest.
To begin the event, open up the latest level you have unlocked in that city (it may be the level that appears before the black and white starting flag icon).
Note:
- If you are just about to build a monument in your current city, be sure to build this first, as this may keep the event levels from appearing correctly.
- If you are within the last 10 levels within your current city (110-120 in most cases), the Marathon will not appear until you've opened up the first 10 levels of the next city in the game.