Go in on Monday, explain your situation and just say you won’t be able to get it done. It takes courage but a much needed courage. Our management believes we can do so much with so little people but what they don’t understand is we had people working late nights and weekends doing 5/60+ hours a week and not saying anything about it. So it gives them that sense of “this much work can be done from 9-5”.
Also remember that even if you’re working 60-hour weeks, someone will just increase the volume of work to require 80-hour weeks and you’ll fail anyway.
Also, longer hours do not improve productivity for work requiring deep focus.