Perhaps use 3 times a week at your 12 mile return leg for high intensity training rides and just chill on the rides in on those days. On the off-days, find a longer route that allows you to go at a more moderate pace for yourself, say 16-18 mph, but double the length of the ride out to 24 miles to start with. Over time, build in a longer "distance" ride once or twice a week and use the HIT rides to build up speed.

It's gonna take a schedule, proper rest and good nutrition/rehab to make it work long term. Otherwise, you're gonna injure yourself in some way shape or form.