As a 70 year old, as you’ve said, your recovery is the main constraint. As such, I would seriously consider dropping the kettlebells completely and switching to a more efficient strength training protocol and combining that with zone 2 and maybe 1 interval session, depending on your recovery. Heavy Squats and Deadlifts demand a lot from the body and CNS in recovery, but are worth it.
I’m sure you’re fond of the kettlebells and they can give you a great workout, but in truth, they are not very effective for getting strong, and the added intensity could be re-deployed in a more focused way.
This book is really useful, I think.
And I know that you’re in a different league to the couple in this video but the principles still stand and it counters the common belief that you can’t get strong or build muscle in later life, which is when it is most important (as you know already).
From my own experience, (I’m 45, fat, and never been any good at cycling, but race dirtbikes), two years ago my knee was in a bad way with damage to the cartilage under the knee cap. I walked with a limp, I had to navigate steps one at a time. I lost 2 inches around my quads from lack of use due to the knee. Then 18 months ago I started actual strength training and although the progress was a bit up and down due to a few niggles and illness, my squat went from a very conservative 155lbs/70kg for 5, to 350lbs/150kg for 5, and 405lbs/184kg for a single.
My knee is basically 90% now, and doesn’t ever cause issues in the gym. My endurance on the dirtbike is significantly improved through being stronger, and my cycling power has gone up about 10% despite not actually doing any training.
Point being, a properly constructed strength program will benefit every area of your health and fitness.