`15mph is a good speed for someone just coming back.
I have been on organised rides that are doing 11/12 mph and I can just about keep up.
I have come back after 4 months off and find that I need to lose the weight and rebuild the condition I had.
I would be so impressed if I could do 15 mph over 20 miles tho mind you I ride an MTB but even on a tourer before my accident I was averaging 12 - 13 so you are doing really well after so long off.
Praise yourself for building up to that speed over a fair distance and work on it till you can do 20mph over 20 miles then maybe start thinking of doing it over off road conditions.
What about swimming, could that help build condition?
I did 20 lengths today and thought I would struggle riding home but in fact felt really good and motored home.
So it will come, maybe use an MTB on rougher terrain, the sheer fun of it will mean you take your mind off the miles you do until you get home and amaze yourself with what you manage.