Hi Severino - I am in a similar position to you, but a few years down the track from an EA marriage. I'm 50 too and wondering how that happened. Have 2 dcs under 10.
I like what noego has just said. It is the process of getting 'psychologically free' that is the hardest bit in some ways. Not sure you have to go down the polyamorous route, but hey - hats off, she's definitely not lonely and I envy her chutzpah in going out there and finding love.
Having young children is lovely and also consuming. For me it is also something that stops me forming another relationship, because it is so easy to have that in the back of your head as an excuse. OTOH, I am lonely and would like to have someone for me - just a boyf, nothing madly serious. Don't want a new father for my dcs or to start picking up socks again, no thanks.
Your marathon sounds like a great plan. And runnning clubs (apparently) are really friendly and welcoming. Also, do you have a parkrun near you? Don't know how that fits in with childcare, but they are really good to do and generally loads of parkrunners peel off afterwards for a well-deserved coffee. Try that.
Once you start marathon training you might sleep better as well! And poor you, insomnia is pants.
Also, while I'm giving advice that I should be taking myself - have you thought of the Freedom Programme?Your OP said he was 'mildly abusive' - you don't need to minimize what you have been through. Everyone on here says the Freedom Programme is brilliant. You can do it online, but doing it with a group seems to be even more fruitful. Think about it.
Solidarity to you.