Fatigue

After years of mostly online chess, I have started playing OTB in a club. I can play well enough for my level in blitz/rapid, but when I am in a classical game I struggle with staying focused, and at around the 2 hour mark I usually end up making a silly mistake that I would not have made had I been paying attention.

Is this something I'll eventually get used to, or is there a way to overcome this?

IMO it's the norm to struggle to keep attention after long hours of fighting chess. To be fair, staying focused for 2 hours straight is a remarkable achievement. In comparison, high school classes typically don't last more than an hour because students are not expected to keep their heads straight after 40-60 minutes. And high-schoolers are generally better than middle-age adults at staying focused.

I used to play lots of OTB classical games until several years ago. Usually my focus level plummets after like 2½ hours of playing. My club mates have similar experiences as well and I'd say it's universal.

Drink lots of water during the event. Do regular cardio weeks before tournament (in fact, make it a lifestyle, since it's healtyh). If possible, don't be old.

Stay hydrated is a good tip and don't have a big meal just before playing.

Dude, I have played in tournaments where a game lasts about 6 hours!