hobkirk, congrats on beating your Cancer!
You are to be commended for that alone, and wanting to improve on your bike just proves that your are a fighter.
I agree with others that losing weight will help quite a bit but it is tough to do once you reach certain ages, like us 50+ riders. If you really diet and try to take it off too fast then your riding will likely suffer.
I've been cycling two years and what helped me get faster, besides doing endurance rides is riding with "experienced", faster riders and really pushing myself when I felt good. I tried intervals and it just did not work for me, but I just don't push myself when riding solo like I do when riding with other riders who are better than me.
Finding out how long it takes your body to recover along with proper hydration/nutrition also helps. For me, that took some experimentation and I have found it does not hurt to change your routine every once in awhile.
I'll be 60 later this year and have gotten to the point of being able to do rides with a small group of experienced riders (3-6) in the 20-21 MPH range for overall average if I'm feeling good and the weather is not too bad. Also talking about distances of 40-80+ miles for some of those rides. But that took a LOT of riding and I'm healthy for the most part!! Currently 6' and 180-185 pounds. Also have the advantage of being able to ride outdoors 12 months a year.
If I push it in a solo ride and don't average at least 17.5 MPH then I'm having an off day. So is it possible, yes for me, but take your time and listen to your body and of course your doctor.
Good luck