If you want to be faster, you have to ride fast. This, is how it was explained to me recently. I know this seems overly simplified, but the truth is, riding faster takes a serious effort that'll push you beyond your comfort level.
Take a serious look and break down each ride. What is a comfortable pace? What pace is uncomfortable; a speed that's hard to hold for any distance. Then ask yourself......are you really putting in enough effort to ride faster?
Except for the surgery, your pretty much described my story........ Started last year, 56 years old, weight was 220 lbs and currently 190, average speed between 15-16mph. Except I have apx. 3600 miles on the bike and another 700 commuting miles.
I too, would like to ride faster at distance, but that's going to take a lot more work and effort. I need to embrace the pain, continue to push my speed to +20 mph and try to hold it and continue to rack up miles. I was also advised to include intervals (on flats and hills) on a regular basis to improve speed and strength.
To be honest though, sometimes I just don't feel like it. Don't feel like burning my legs off or running out of breath. It hurts, I get sore and it just wears me out. But I know, it's what I need to do if I ever want to increase my average speed.
Well, that's my 2 cents......all the best.