Sorry you're here. Hopefully this site can help you learn all the things I wish I had known so soon after DDay. (I didn't discover it until three years later.)
For starters, don't assume he's not still in contact with his AP in some way. Don't assume there aren't other APs as well. Don't assume there haven't been other APs in the past. Don't go to MC yet. Don't ever let him blame you or the marriage in any way, shape or form. Don't ever do the pick-me dance.
Definitely go to IC--both of you. Definitely figure out his "whys" (it's never "AP was attractive and providing attention" or "I felt neglected." It's always "my mom never loved me" or "I've felt not good enough my whole life.") Definitely assume you only know about 1% of the infidelity at this point in time.
I was 34 with babies when I discovered my FWH's double life. I know exactly how it feels to see your dreams of a happy, stable family life get ripped away from you and not know what to do. I was resigned to D the day I found out, but my H was acting so remorseful and desperate to save our M that I decided to try--I also was not wanting to break up my family and change the trajectory of my children's lives.
Little did I know that while he was putting on his theatrics (ie begging me night and day to not leave him, calling in sick to work every day to stay with me, buying me a new, very expensive car and designer purses) and giving me full access to his life, he was still talking to one of APs and hinting that we might D, thus giving her what she wants.
I fought, I screamed, I shouted, I threw him out, I searched through all available data, I called the APs, I made him seek revenge on them on my behalf (eg calling their jobs to report their behavior)--basically, I was not passive or open to reconciling or letting him off easily. I was prepared to walk at any moment. He now says that knowing the person he used to be, if I had been passive in any way, he would have taken complete advantage.
Now, five years later, we've been through hell and back again, but we are firmly in R. He understands his whys and is proud of how much personal growth he's had. It was a long, hard road though, and if I had to do it all over again, I don't think I could. But I have my family together, and that's the most important part for me.
[This message edited by Revenger at 8:01 PM, Wednesday, September 11th]