If you do it as a free ride, it has a 5.6km lead in and then you do the 15km circuit. 20.6km
However if you do it in a race, the lead in is longer because the start point is different (the pens are farther away from where you start in a free ride). 24km.
The lead in includes the Mayan Mountainside KOM so you end up doing that twice in a race, plus the Itza KOM once right after the Mayan Mountainside KOM ends.