I thought of a few things that might be helpful to those about to have mastectomies.
It's probably gonna be nowhere near as bad as you fear. No-one I've spoken to who's had the operation (can only speak for no reconstruction) has had any significant pain afterwards.
Stay ahead of the (probably fairly mild) pain by taking regular painkillers. You'll probably be given the equivalent of over-the-counter ones, and this is likely all you'll need. I took them for three days religiously, and just as and when I started to feel ache-y after that. Trying to lie in unusual positions to sleep probably caused more discomfort, as did the sticking plasters on the drains.
Drains. You'll have a soft silicone tube running from below the wound (probably out of a little hole below the scar) and into a little suction pot. Get a little cross-body type bag to put the pots in, or wear things with pockets that can hold them. If not, you might get them caught round your ankles when you go to the loo, and rip them out when you stand up and have to rush to A&E in your pyjamas to get it sorted. 
My drains stayed in for 10 days. Oh yeah, try and have a little table or pile of books by the bed. The drains can't be above you, but if you sit them on the floor there's not enough slack in the tube (depends how high your bed is I suppose).
You won't be allowed to get the scars wet for 7-10 days so bathing and showering is out. As I wasn't really mobile enough to safely get in the bath where our shower is, I got one of those large rubbery bucket things to stand in, and tip jugs of water over the bits below the scar. It was a lifesaver being able to feel properly clean.
Definitely do the exercises they give you - mostly gentle stretching to stop the scar tightening, and preserve your range of movement. And if they don't give you a sheet with the exercises on, ask because they should.
Don't be terrified that your first wee after the op is bright (and I mean really bright) blue. This is normal, and to do with the dye they use to help them trace the path of lymph fluid during the op.
Don't forget to ask for the tinsel-lined bucket to take the boob away in. Just coz KeepCalm's consultant said no, doesn't mean they all will 
purple I hope you get home soon. And that someone is bringing you chicken soup... or the vegetarian/vegan equivalent
Leslie Sending you love. I''m sorry you're locked in the tower with all that wool and two knitting needles. If that little man comes back, tell him his name is Rumpelstiltskin and you'll be free!