My wife & I were practically in your exact shoes back in 2010 ... 5 years away from retiring (with over 25 years of hardcore backpacking under our belt), and then wanting to do the AT for a retirement kickoff. We've been accused of planning things way too far in advance, but we were only 50 when we quit the jobs, and every one of our accusers is still trying to pay off their mortgage.
It's pretty darn nice be a long range planner!
We did retire right on schedule, and we did start the AT ... but quickly found out that the AT was the exact wrong thing for us at that particular time. There are numerous reasons why we bailed on the trip, but it basically boiled down to the fact that we ended up hating almost everything about East Coast hiking. We're from the Central Interior part of British Columbia, and our home backpacking territory is basically Jasper & west into BC ... pretty different from the AT. I'm not trying to start a troll war here, but for us that was what it was. It only took us 2 weeks to figure it out on the AT, and we're pretty good at making big fast decisions so we bailed. No regrets. We came back to BC and had the spring/summer/fall backpacking season of our lives.
One big time recommendation would be to plan a two week vacation to hike the Georgia to the Smokies section ... a year or two before you retire, and at the exact dates you plan on starting the AT. You'll find out a bunch of stuff that you might not have thought of, and conclude the AT is just not your thing. We knew a bunch of facts from research, but that only gets you so far and very little of that is emotional. I was surprised by how much of a negative vibe my wife & I got from not seeing a single animal track on the AT in all of Georgia ... not something that would even be noticed by most people I suspect, but we are huge wildlife fanatics and Georgia left us feeling somewhat hollow. The other shock was just how bad the water sources were compared to our baseline thinking of water ... while most people we met were saying that the water sources were doing great that year because of all the rain. We did know from research that water was going to be different, but the reality hit us as a big negative vibe that brought us down. These two relatively minor things (and a few others) made us start thinking about what we could be doing the Canadian Rockies in the spring & summer instead ... and that basically sealed the deal on our bailout.
So do your research, but get out to the actual trail and do some hardcore testing of your gear ... and your brain! I hope it works out for you, and you end up on top of Katahdin ... it's a worthwhile physical endeavor for a lot of folks.