I would suggest:
1) see your Dr. first and get the all clear
2) continue what you are doing as a base and increase your weekly mileage by 10% each week. Three times per week.
3) as you get stronger, allocate the additional mileage equal to the 10% increase to any or all the individual rides during that week.
Do not exceed the 10% mileage increase per week until you have doubled the initial starting base mileage of week
#1 ie: after 10 weeks
4) continue to increase weekly mileage until desired distance is achieved.
5) Have fun doing 2) to 4).
You will eventually get there.
Good Luck