Ken,
We have very similar goals. You can absolutely train into an endurance athlete. Exactly 3 years ago today I bought my first road bike at the age of 53. After training with local club, and a lot of solo “sweet spot” rides outside, I was able to complete an event with 15,000 feet of climbing on 5 mountain passes over 117 miles. At my weight (204) and ftp (~260), that meant 8 hours of climbing in a single day!
A structured base/build/speciality plan like TrainerRoad will likely deliver quicker results than semi-structured riding outside. Before signing up for TrainerRoad a year ago, I was following Strava/CTS plans outside. I made plenty of mistakes, but was able to bump my ftp up to 277. I think 300 is possible by following TR plans, unfortunately this past year I’ve been rehabilitating and old knee injury (ski accident almost 40 years ago).
Hope that helps.
Brian