Yep, that is how it starts. Running long distances with people, especially with people of the opposite sex, builds intimacy. You share hours of time, your share pain and joy, you have build memories together of all sorts: the dorky guy that you just passed, the dog that chased you, the sting of sleet, perhaps the beauty of a sunrise. Personally, I don't think it is a good idea at all to train with someone of the opposite sex. It just isn't something that will help a marriage. And if you told him it bothers you and he still won’t stop that tells me that he values his time with her more than he does his marriage. And that isn’t a good thing at all.
With that said, I think you now know why he really left. You started to interfere with his emotional (at least) affair. So he wants to separate so he can continue this relationship without you there making him feel guilty. Most people don't separate to be alone, they just don't.
Perhaps Puppy or Gucci will come along and give you a plan to shake things up a bit. I'm not the best at plans at the moment.
It is just too bad that you can't train with your husband and start to build intimacy with him. I am a much faster runner than my wife, but I will slow down to her pace and run with her when I can. I just slow down and we talk. Imagine talking with your spouse about something besides bills and kids and things that are broken!
I'm a man . . . But I can change . . . If I have to . . . I guess . . .
