[dyno12345]

Tell them that in exchange for 20% more time you expect a 20% pay raise. Talk about all the things you're going to have to give up if you take that extra time, and how that makes you unhappy. Since they're asking for something from you, they can't get mad at you for negotiating something in return. No one can call that unfair with a straight face. Most likely if you do this they probably won't actually give you 20% but will may offer you something much smaller. You can decide to either accept this, or, you can say that their counteroffer is not good enough for you, so you can't take the extra time. That way your reason to refuse is completely impersonal, and not because you don't care.

Regardless, start looking for another job in the meantime. The culture of where you work sounds completely inhumane.