[herr_oberst]

This sounds like a lot of fun, and it will help you transition now that you're retired. Your idea of commuting to the dog walk is a good one. Any base miles will help.

Keep us posted and we'll help you get this century under your belt!
Are you exceptionally tall? a 66 is a big bike. (A 59 is a big bike!)

The only advice I can add right now is that I don't recommend flat bars. At all. Not enough hand positions. Modern drop bars are well designed to give you many comfortable options while you put the miles in the bank.

If you want to have some fun, go to several bike shops and ride some bikes. You don't have to buy a new one but you might find one that has a comfortable fit that you can try and replicate with your new frame.