Bill, I'm getting to this late, but I don't see some of my points mentioned so I will endeavor to offer some explanations for your observations.
Even at our advanced age, the human body is a fairly adaptive mechanism. Soon after you start an exercise regimen, your body realizes that this is the new norm and adapts to it. You get an initial improvement in muscular strength and in the body's ability to use oxygen more efficiently, but the body doesn't adapt more than it needs to in order to accommodate this new level of effort.
But, what this also means is if you use the same exertion for the same period of time to reach a certain level of exhaustion, you will continue to reach that same level of exhaustion after each similar period of exertion. Your body won't adapt more than is necessary.
This means, if you want to achieve increasingly greater strength, endurance, and cardio-vascular health, you need to regularly increase the effort-level/effort-time of your exercise periods. If you have been walking along at 3.5 MPH on a zero incline for 30 minutes, increase to 3.8 MPH on qa 2 incline for 45 minutes. And when you can do that without needing a nap afterward, increase it again.
Of course all this comes with certain purely physical and undeniable limitations--at our age, more limitations than we would care to admit. Arthritic joints, stressed tendons, arterial blockages and all the other ravages of time tend to adversely impact our abilities to physically improve--as does the simple fact that recovery takes longer than it did when we were younger.
As a young man, I used to hit the gym for a couple of hours 5 or 6 days a week. That is no longer possible. Bad knees, prosthetic hips, and a shoulder that needs replacing (surgery scheduled for Oct. 13) have all contributed to a reduction in gym time. Still, I hit the elliptical three days a week for a 45-minute workout. That allows me to remain in a state of physical fitness that is in concert with my overall activity level. I am no longer the buff beach bod I once was, but I don't have to turn out the lights when I take a shower.
To put it in perspective, Bunny used to say, "Honey, you look like my Greek god." Now she says, "My god, honey, you look like a Greek."
Age happens. But, lest you think limitations are only visited on the old, there is a story about a young farm boy who decided he wanted to be the strongest man on earth. To reach this lofty goal, he found a newborn calf and lifted it every day. As the calf grew, he reasoned, his strength would also grow. One day, he thought, I will be able to lift a grown cow and no one else can do that.
Well, if you have ever spent any time on a farm, you know that a calf grows faster than a healthy farm boy's ability to increase muscle strength, and a grown cow is too heavy for even the strongest man to lift no matter his preparation.
So, with all this as preamble, continue on with your exercise program. It's doing you good even if you can't see immediate results. Add in a bit more challenge every so often, but don't go nuts over it. Think of it sort of like the guy who quits smoking. The physical repair of his body is so slow, he may not realize that he's feeling better each day. Until, one day, he finds he can walk up a hill without wheezing.
FWIW,
Crusader Rabbit