Yep, other than getting seat time in on the local hot days there is not much one can do otherwise to adapt to long hot days on the bike.
That being said using long training rides in the heat of the day to determine which kit is most comfortable, what sun screen/lip balm is effective and how to carry a reasonable H2O load is realistic testing.
Not much fun to find that a jersey sags uncomfortably w/ two full water-bottles stowed, there are hot spots from mile 50 in the shoes and the new sun-sleeves are a chaffing annoyance during a LD timed ride.

We S-TX locals get stared as early as practical, stop as little as possible and get LD rides over with before the heat of the day bears down.

Good Luck, be Safe & have Fun.

-Bandera