I have been exploring the same option and several others. My wife rides, but has not interested in riding across America on roads. I also have a few concerns about her safety alone in the desolate areas along the Southern Tier, her boredom, the lack of communications between rider and SAG (no cell coverage), and for me the heat, and the long distances without services. I too am in my 60's, even older than you, and did a lot "back in the day" and we do annual 7-10 day bike treks (Pre-COVID). Pounding out back-to-back centuries takes its toll, and it sounds like you are planning on more of a speed ride than a tour.
I would ask, what is your WHY? Why do you want to ride across America. I have been struggling with that since it is one the last items on my adventure bucket list that requires physical stamina. I have been looking at the Rails-to-Trails route, thinking about using two cars and a travel trailer and section riding with my wife from coast to coast. My current decision is to ride solo on the Trans America and take my time and tour.
I used Google maps and virtually rode over 1,000 miles of the routes and spent a long time imagining riding each route, day-after-day in heat and rain.
Several years ago I thru-hiked the Appalachian Trail, 174 days, 2,175 miles. There are days when it was like having a job, and you had to walk 15 miles whether you wanted to our not. I started out as a speed hiker and met an older hiker who had done the trek before and he gave me some sound advice -- Slow down and enjoy the hike, I will probably never get another chance to do this again. Savor every mile, take in every view, there is no trophy for finishing fast.