Stay away from Ashley Madison.  99:1 guys:women.  And those women are mostly bots.

First, I agree with Lester:  AM is a waste of time as is AFF.   TER is real of course.  And the good thing about TER is that the providers couldn't care less if you are married or not.  On SA some of the SBs will rfuse to date a cheater, some really don't care.  You just need to be straight with them and not try to lie about your status. I also agree that tinder is a bust for older guys seeking younger fems for an affair. Then there's OK Cupid.  It can be a PITA to navigate their site, but people on there seem to be serious about meeting up.  You will only be taken seriously if you answer lots of their questions, which determine your suitable matches.  It's kinda fun once you get into it.  Just be clear in your bio that you are married but looking.  

I'm assuming from your post that your wife is unaware of your dating activities, so once again, I'm gonna preach my oft ignored advice about marital honesty.  Why not tell your wife you've been craving some additional feminine companionship in your life? Explain your love for her, your commitment to the marriage, and your desire for her permission to occasionally date others?  It worked well for me. My awesome wife and I now have lots of fun talking about our affairs!

I think you have to know your wife pretty well.  Telling her would never fly for me.  I just know how she'd react.
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This actually goes for a lot of things in marriage ... things you learn never to mention.  Sometimes they are the simplest day to day things.  But mentioning them triggers the listing of oft repeated grievances.  I get with innocuous sit that happened 12 years ago, as if it just happened 5 minutes ago.  Anyway, I digress ...  
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Honesty isn't always the best pollicy.

I had a very simple solution, DIVORCE. you know why divorce is so expensive? Because it's worth it. Or at least it was for me.