Sitting in a sofa for hours is terrible

So i'm staying for three weeks as a guest at a household were they are all extremely nice but also extremely sedentary. All they do is sit in the sofa for hours watching TV and stuff. I'm not the most active person at all but I very rarely sit in sofas/armchairs, I usually sit on hard chairs or computer chairs and never for SO long. I really feel the difference. It's only been five days and already I'm noticing changes in my body, and not in a good way - my belly is ballooning up and my posture is changing - I'm not even exaggerating. I try and get away for an hour walk every day (more than that feels impolite) but it still doesn't feel enough. It really goes to show how bad sitting on a sofa for a long time actually is for you, or at least it feels that way to me