Your situation is almost exactly what my W did. Hers was with an old BF she knew before me. It lasted a few weeks of heavy texting and phone calls, during vacation too. All started "innocent" same as you. He started with the sexual innuendos, seductive talk, probing for where it could go. She liked the attention and the compliments. Blah, blah, blah. When I found out she lied about everything. And kept lying, and may still be. It took 6 months to get an admission she even met the guy even though I knew she did. Then 2 more months when the OM sent me a pic of another meeting they had. Again denying she met him. And 2 years later we are still together I don't trust her fully and when I think about it I get angry.
So whatever you do....DO NOT LIE! Tell the whole story, don't leave anything out, answer all his questions fully, don't downplay what you did. If you want him to trust you, you have to be trustworthy, little things and big things.
Realize that you are responsible to rebuild the trust and the timeframe is how long it takes him to trust you. The length of time will be partially dependent on your actions and partially about him. You can only do your part. Actions are better than words.
That is one part. You also have to explore yourself and determine why you did this and fix your issues. If you have some low self esteem issue and need the attention from OM's then you need to fix that or you may do it again.
Me:49 W:45 M:19 T:22 EA confirmed and ended 8/2014 S:19,17 D:9,5