Fitness aside (and they must have been supremely fit), they must have had good water supplies; but even so, the risk of heat exhaustion under armour in 34C of heat or more must have been extremely high.
As a follow-on question, would the Romans have fought in the middle of summer if they had the choice? I've heard about armies coordinating warfare around different times of year or events like winter and harvest time. Is there truth to that and if so would mid-summer have been a time the Romans would have wanted to fight?