My best Friends Mom

I'm Russell 23 average type, nothing special. My friend John 24 joined the Army to follow in his late father's footsteps. His Mother Karen was a good looking woman of 49 with red hair and a Doctor by profession. John's father had died in a ultralite crash 5 years ago, I was asked by John to help Karen around the house like mowing the grass, trimming hedges and such. He said he would pay me, but I refused saying what are friends for,his siblings lived out of town. I would go over on the weekends and tidy up the yard and anything she needed done. On a Saturday a month ago I arrived and started by shovelling the driveway. Karen was sitting inside looking thru albums and drinking. After finishing the work around the house, I went inside asked her if she wanted anything else done. She replied "Yes there is 1 thing you can do for me". "What is it" was my reply. "You can fuck me, I'm so horny and I haven't a fuck in over 5 years". was her answer. "You've been drinking quite a bit and you will probably regret it later" I replied. "No I won't, I've drinking to get up the nerve ask you and I don't want to out and get picked up at a bar by someone.I would rather it was someone I know and like". was her reply. I didn't know what to say to that. Then I said " If John was to find out it would end our friendship". I replied. She said "I'm not going to tell him or anyone else either, it'll only be you and me that knows". I said "If you won't hate me later, Okay". " Good let's go to my bed". She said. We did and we had sex twice. The next day I returned to Karen's house to see how she was. Expecting her to be angry with me, but she welcomed me in and said "Thanks for yesterday, I really needed that and if your willing. We could do it again and I've had nothing to drink today". I agreed and we spent the day together. We now have had sex 7 times and it's just sex, I'm fucking Karen more than my Girlfriend Cheryl who I have asked to marry me last week and she agreed. She doesn't know about me and Karen except about me doing odd jobs for Karen. She even comes over sometimes for a while talks to Karen while I'm working outside, they get on great.