Karen is likely right on target as usual, AnnieGirl, and I might add that
low levels of vitamin C will cause symptoms. Not very many doctors
check for that; often they check for B12 levels because the B vitamins
all provide for increased energy, but they do not always check vitamin
C or D levels.
I'm not a doctor and can tell you only what has worked for me. When I feel depleted of energy, I look back at the day prior to the low-energy- level day and make sure that I didn't have an overload of heavy physical activity at that time. If not, then I know that I should increase my vitamin C, D, and B levels by taking a high-quality multiple vitamin. In addition, adding some good quality protein to your diet might help the energy boost. As you know, many people take a high-quality multiple vitamin daily just for good measure because of the American diet.
Metabolism slows down as we age, and it may be that's also what the
doctor was referring to. The treadmill will help with energy production, too, so I'd keep that going if you feel like it.
You've had all the allergy tests, I feel sure, for foods and inhalants, including trees, grasses, molds, etc. This is the grass season and bothers those who are allergic to them. You might wish to notice whether yours is an all-the-time reduction of energy or seasonal. If seasonal, you know you have an allergy of some sort that hasn't been verified yet. The two most common food allergies that cause reactions may be corn and wheat for many. For others, it's something else in the diet.
You have a puzzle to solve, AnnieGirl, and that's how you might wish to investigate your evaluation of what's happening. Make sure of Karen's suggestions first, however, because she is moderator of Fibromyalgia, Allergies,Depression, and Chronic Fatigue Forums and knows the way to guide you in your search.