Congrats on your breakthrough hobkirk. It just goes to show you if you just keep your head down and doing the work sooner or later the results will come.
Like you I came to cycling from a running background. I never ran a sub 3 hour marathon but I have run more than a few and qualified for and ran Boston. I've had achillies reconstructive surgery that required a tendon transplant and a hip replacement as a result of a fracture that put an end to my competitive running days.
I love to train so when I made the switch to cycling I pretty much adapted what I did as a runner to the cycle. Intervals twice a week including one session of shorter more intense intervals and one session of longer threshold repeats. I try and do at least a 50 miler on the weekend. I use a powermeter to gauge intensity since power is a more accurate metric than heartrate for judging intensity. In my case even more so. I had an issue with an arrhythmia that required two cardiac ablations to correct. It took me a long time to lose the fear of a repeat episode but I think I have started to remember how to suffer again;)
I really identify with your quest and plan on pushing as hard as I can for as long as I can and not make any concessions to age.