Training for the heat begins about 2 weeks before you enter that environment. About an hour a day of introducing deliberate heat training (overdressing, outdoor timing) for 10 days is enough to acclimate. Otherwise, the advise above and starting as early as possible in the day.
However, if your 70.3 event will be hot, this is the time not to avoid, but embrace the heat to prepare. You don’t need to do the entire workout at the hottest part of the day, but may time it so that an hour is relatively warm.
David