You’re meant to draw sitting upright with good posture for your health and to make it easier/mark correctly but it is so tiring.

You need to be aware of your posture

art is a physical activity that takes hours at a time, you need to have a posture that reduces muscle strain. Pivot from your shoulder.

Look at videos on YT by artists about how to work and how to set up your work area. Mark Carder has some good videos.

and the stamina required isn't just physical, it's also mental.

I tend to work in 4 hour blocks, work for 1.45 then I take a rest, walk, stretch, have a cup of tea, then work for another couple of hours. More than that and I am wrecked

There is a reason my favourite statue is of Watteau, a friend depicted him at work in his studio, you can see his stance is one of relaxing as much of his body as possible, he can look at both his subject and his canvas without much effort but you can see the concentration in his face, his shoulders are relaxed, he stands like someone who stands all day, shifting his weight from one leg to the other , resting an arm on his hip

https://api.art.rmngp.fr/v1/images/17/211803?t=cziuTfdlJ0G4CxdKApn7VA