Congrats on your progress from a fellow 68yo! Only I’m a single TKR, and mine is 21 months in place. If you haven’t seen it yet, lots of good discussion in this thread. I can only speak from personal experience and not as a medical professional here. However, I’d suggest being only 5 months out now from your most recent replacement and roughly 9 months out by the time of your target event that you may be a bit ambitious. It’s possible you could risk overdoing your training and actually setting your full recovery back. Most Orthos say your knee needs a minimum of a year to recover, and more likely as much as 18-24 months. Others here have their own experiences with which they can chime in.
Kelly Starrett has some comments in his serious reference in my last post in the referenced thread that you can’t accelerated the pace of healing beyond certain limits. Don’t want to be Debbie Downer as I get it totally on wanting to prove people wrong and be the exception who recovers faster and can do more sooner. Tried that… learned something. I think my wife calls it “patience, stupid”.