I've bonked badly in a hot marathon being used to train in the 10-15 degrees of the UK weather too, but it's not even going to be 18.
Also heat acclimation needs 2-3 weeks, not months and months
, unless you are doing an Ironman in Kona in 30+ degrees, in which case fair enough.
On a second thought, it's rather impossible they'll run Valencia. Tits is badly injured and even if he recovers he will have effectively missed the most important weeks of marathon training (yay stupidity) and Kels has just run a marathon PB. In the best case scenario she will need 2-3 weeks to recover because no matter what she says, that was an all out marathon effort for her. Just more appropriately paced (but still slowed). I can't see her reproducing that in a few weeks.