I am feeling very sad for you.
I am not getting initially no one thinks about future , becz in enjoyment all brain gets stop working . Let it be , now from many years you have enjoyed her hole , now you are thinking about your future ? And whts wrong in her demand ? She fulfilled all your wishes and in return she is expecting such demand because she also worried about her future like you ,
Now your present situation is really critical , because if you say no to her then she may blackmailed you for your relations , we can't throw possibility that she may complain against you that you have raped her .
So all this can happen if you say no .
If you say yes , then your age gap is there which will create problem in future .
Both you kept sexual relations only to full fill each others phyr needs. No love is there between both of you , so surely one day comes that you will get bored to her body , if love was there from many years then sex between 70 years old lady and 45 yrd guy is pleasant to each other , but between you such love is not there.
Now do one think , just talk to her , convince her that you can't Marty her , explain her that you will be there throughout the life with her in all the needs. You will take care of her. Convince her that even after your marriage to other girl , you will be there to satisfy her physical needs.
Remember that she is demanding marriage because ahe wants to secure her old age life so give her assurance that you will satisfy her physically as well as economically.
And after she convince marry a girl of your choice .
But threat of blacailling will always be there with you
From your story / question , this is great lesson to all those ladies who are in sexual relation other then marriage .All the ladies friends , remember that men just use ladies for her hole , when time comes to take responsibility then same men hide under the couch .
So be alert,