It’s sweat, I’m not sure how much sweat marathon runners sweat out during a race, especially since many of them dehydrate before the race to lose some weight but I wouldn’t also bet on a marathon being more taxing than an F1 race its both are very hard activities as driving a formula car takes a lot of strength.

“Believe it or not the driver has to apply over 930kg of pedal load per lap in Canada - an incredible 65,590kg over the course of the 70-lap race - and it’s all down to the Circuit Gilles Villeneuve’s unique blend of high-speed straights and tight bends.“

https://www.formula1.com/en/latest/features/2015/6/under-pre...

It is water weight being sweated out. Largely because it's a very taxing activity.