I had the conversation. W came home from work and the house was empty as kids were at my parents already. We said hi to each other, then she told me she was heading out to hang with her brother. I asked her to sit so we could speak for a while, and she bristled and rolled her eyes: "oh come on, what?". This is the exact reaction I was expecting, and I didn't respond. She opted to stay standing and said "What is it?"
"W, we've have been disconnected for a long time and it doesn't work for me. I am going to file and proceed with the divorce."
She was equal parts annoyed and angry, but mostly annoyed. She came back with all the normal responses she has used before: people go through rough patches, I won't do that to the kids, you're just being selfish and trying to control me, you're rushing me, all you care about is whether we're having sex or not, etc.
I did my best to validate and agree where I could. I kept the focus on the facts, that our relationship was very disconnected and had been for many years. That the drinking and wild behavior was not okay with me, that I won't accept that for myself. One of my goals was not to allow this to turn into an argument, and so there was no arguing from me when she tried to start one. I didn't want to rehash things we've talked about ad nauseum, this was not the time for that.
Only once did I mention the affair. She said I was overreacting to a basic rough patch and I said "W, there was an affair and I was lied to for many years about it. This has damaged the trust in our relationship, and there has been no effort to repair that trust."