If you're fit and stay that way, the age won't be a problem - our dock-neighbors are (finally) hanging it up - he's in his mid-70s, and their selling their boat not because of their ages but to spend more energy on her music!
As for staying motivated, I suggest the best thing to do is take classes, lessons (BOTH of you), sailing vacations, read, read, read - keep the ideas in the forefront.
I'd be much more concerned about the fact that wife doesn't sail - you're planning a pretty long passage, if you can't count on her to take the helm at some point, when will you sleep? Will she stand watch? Does she even share this dream?