Hey /u/thingsorfreedom. Just read some of your backstory and I think I'm in a pretty good position to comment on your situation. I'm 48M, separated early 2014, divorce finalized late 2014, one kid in elementary school that I have week-on/week-off custody for. I'm more of a cyclist than a runner, but also in good shape, etc. In my case, the divorce wasn't really my decision. We'd had dead-bedroom issues for 3-4 years, and then I found out my wife was having an affair. For me the emotional divorce happened about a year and a half ago - that's when I took off my ring - and I've got some direct experience with most of what you're asking about. So ...
First, the "do it for the kids" debate is tough. Very tough. The first few months after we separated I would break down crying at the thought of only seeing my son half the time. It sucked hard. But I've come to realize (or maybe it's "rationalize") that all that's really happened is that the 50-50 custody we have is a more formal and efficient version of the parenting back-and-forth we were already doing when we were together. I actually kind of enjoy the predictable change of pace these days.
But don't for a minute think divorce isn't hard on kids. No matter how civil it is, no matter how well you and your ex get along, your kids will pay a price. E.g. I want to sign my kid up for soccer, but that involves coordination with my ex since she'd have to take him to half the practices, and conversations with her are emotionally "challenging". And we're not great at coordinating what it is he needs to be studying over the summer to be ready for school next fall, so he'll probably be a little behind where he should be. You bump into little shit like this on a weekly basis and it stings every time. And then there are the joys an unconsolable 5-year old who, for whatever reason, thinks divorce means he's never going to see his mom again... yeah, that one's real fun. :-(
But do kids gain anything? If our pre-divorce home had been filled with anger and hostility I'd say unequivocally, "yes!" But my ex and I actually got along pretty well, we'd just drifted apart. So it wasn't like my son was being raised in a particularly dysfunctional family. But he wasn't witness to what a really loving marriage looked like, either, which I think is important for kids to see. I'm not sure there's any way to quantify the impact of that.
The question of whether or not to stay in a marriage for your kids is, ultimately, one where there's no right answer. When I'm confronted with this sort of tradeoff, I've found it's best to just bite the bullet and pick the option that results in the most change. If it doesn't work out, well, at least you've learned something and can make a more informed decision next time, which is more than you can say for sticking with the status quo.
As for your question in a previous post about whether it's too late for you to start over, I'll echo the sentiments expressed by other commenters there. Nope, it's not too late. When it comes to dating, it's a bit of a seller's market for healthy, well-adjusted, 40-something guys.
As for how things have turned out for me, well... I won the proverbial lottery. I ended up meeting the woman of my dreams about a month after my wife moved out. Against all odds (and many people's better judgement, including my own) we started dating and, today, a year and a half later, we're more in love than I would have dared dream of. The contrast between the marriage I had and the relationship I have is astonishing. But it's not without it's challenges. The realization that the person next to you in bed doesn't share that 20 years of history and experiences that you had with your wife is sobering and a little sad. It highlights the fact you're starting over from scratch. Some days that's a good thing, some days not so much.
Divorce is a crucible. It's made me wiser and stronger. I am a better partner, a better father, and a better lover for it. But I'm also a little more jaded, and at the end of the day the one real regret I have is the impact it is going to have on my son. But for that, I'd say I would do it all over again. But that's a big regret. I won't deny I still think about the alternate universe where my ex and I somehow reconcile and rekindle our marriage, and where my son doesn't have to worry about what happens to him if I remarry.