So, I'll go ahead and be the one to disagree. I think most of his points are valid, but individuals are different and what works for Master Yoda isn't necessarily what works for Luke Skywalker, which is probably totally different than Darth Vader.
I'll also throw this thought into the mix... Active Relaxation
an ability to relax parts of your body while others are called to work. It takes practice, awareness, and being "in the moment", but my longevity increased tremendously once I found out I was mostly causing the pain in my butt. Your body will naturally tighten and being aware when it happens and forcing yourself to relax those muscles and not spend that energy created a more effortless ride and I feel more aware for a longer period of time.
Try actively and totally relaxing your elbows, shoulders, neck... then back waist, thighs, knees ankles, toes...work through everything you can. I'm not saying I ride like that all the time. In the mountains I shift weight off centerline, weight the inside peg, rotate the hips and open up to the corner, etc... and every motion and thought process requires energy.
And that is how I measure when I get tired. How much energy do I have? I've done 2 Saddlesores (1000 miles) and a Bun Burner (1500 miles) now and they were much easier than a 425 mile day I did last month through twisty mountains in the high 40's and misty conditions. I was simply required to spend more energy, worry, and concentration in crappy conditions than blasting out mileage on the super-slab listening to some good tunes on a sunny afternoon. Eating smaller amounts throughout the day and remembering to fit in something with electrolytes also helps. The minute I start yawning I'll try to snack on something and it is usually just me running low (I tend to ignore nutrition and meals sometimes in the heat of the moment). And don't underestimate a good set of ear plugs... they make a difference.
The last thing I'll mention is to consider finding a way to test cognitive abilities, and this will be different for everyone. For me, if I can't calculate time/distance, fuel mileage, how far I can go on available fuel, or stuff like that... I've been on too long. Lets pull over, stretch the legs, visit the restroom, and even nap if needed.
I'll also admit that more energy is required to move an old body than a younger one... and let's get into the gym people!!!