Take short, calculated breaks often.
If our physical energy matched our enthusiasm, we'd go all day without sleep for decades.
But we're human, we're frail, and so we have to think smart in the present.
And that includes taking breaks.
If you're filming a long event, take breaks during the lull.
Find out when things slow down & plan your break around it.
A half hour is usually all you need to charge gear, fuel your body with nutrition & a bit of rest - then back to it when everything picks up again.
Of course, have your camera on standby... JUST IN CASE.
Pro-tip: If you have another camera operator on your crew, take a rolling break.
Have them that a break first, while you continue filming.
Then when they're done, take your break as they go back to filming.
That way, there's always someone capturing footage.