I think it's partially just a desire to feel young again on your part. She reminds you of being younger (as it is an old friend you'd forgotten for 25 years). The idea of breaking the routine, etc. Being married for decades and being young and in love/lust are so different, and there is a certain newness and excitement that goes along with it.

I'm sure that you and your wife are very much in love, but I think you need to a. learn to communicate your desires with her and b. add excitement and spice back into your relationship. It's harder than it sounds, and it doesn't sound easy to begin with. Experience new things together, and experience new things on your own, so that you have stories to report back to each other. Take dancing classes, get a new hobby, go on an adventure!

What was it about this woman that led you to feel attracted to her? I believe that the qualities we desire or are attracted to in other people are also qualities that we would like to attain ourselves. Was it just the naughtiness of it all? Perhaps you and your wife could exchange dirty text messages, tease each other a bit throughout the day, maybe send a few naughty pictures! I'm sure she's not happy with you after receiving the information regarding the woman, but maybe once you're on better terms again, this is something you can work up to.