HM in 86% humidity

[Pickynails]

How bad will it be?

I'm well trained for the half marathon, I've done the training over a British summer, so mixed weather, but some hot and humid.

I'm travelling to the Med to do this race and the forecast is for 25 degrees and 86% humidity...It was supposed to be cooling down by mid October!

Is that the forecast high for the day? Generally in warmer climates they start very early so it may be much cooler than that.
otherwise sensible hydration, including the days before, (remembering that over hydration can also be very dangerous), slow down it doesn’t matter if you miss your time by 10 minutes, avoid any alcohol for a good 48 hours before.
And incase the worst happens, make sure your travel insurance covers doing sport, a lot doesn’t.