But accepting that after a couple of very productive hours, you need a break before you can come back and do that again is a good start.
Get outside and just walk. Even if you don’t have the “aha!” moment you’re looking for, you’re probably going to be likelier to stumble upon a creative breakthrough later for having taken the break.
Eat your lunch away from your desk wherever possible. Stand up if you can (standing desks, breakfast bars, and so forth are helpful here). Making lunch a set time each day that you actually take a break is a simple way to make that time within the allowances of a pre-set working day