I can't add much here at all, other than to say you've gotten some sage advice thus far. I might reverse my training and do 4 days on the the bike, and 2 days in the pool. Add in some light core work too. A long day in the saddle will test your core strength, I know it does mine.

We have an 50+ forum here too with some helpful info as well for us older riders. (I'm 53.)

Looking forward, if you stick with a road bike, you might try out one that's got a more upright geometry, like a Specialized Roubaix, or Fuji Gran Fondo. There are a good number of those bikes out there now.