51 is not that old as you are aware but physically things do change around that age and re-thinking your fitness programs, as you are doing, has to be done by everyone as we age. I'm 63 and do home design for a living. That means 2 things. Sitting in front of a computer all day and, this is important, burning TONS of sugar in the form of glucose through using my brain all day. It's my opinion that burning all that glucose is what wears us out after a long day of doing nothing physically.
So what to do? Again only from personal experience, if you can break up that long stressful day of burning glucose with moderate exercise then the sugars get rebalanced in your body and you can work longer and feel better with more physical energy at the end of the day.
I have a small project in the back yard moving dirt and when I feel myself fading from the computer work I'll take a break and go move some dirt. Seems weird and a bit non-intuitive but it really helps me stay energized to tackle the computer work again.
It's not possible for everyone to take that kind of break but anything physical through the day will help. Try it.
Again I think the dog tired you are feeling is genuine, you simply ran out of the body's glucose and need to recharge. Something else I do is drink fresh squeezed orange juice through the day to keep those glucose levels up and as others have said it is, again my shared opinion, that exercise when you are tired from overexerting your stress muscles can really, really help but getting from dog tired to a nice brisk ride can be a monumental task.
I would really like it if you shared your energy level in a couple weeks. It might be just fine and you've only hit a dip in your energy as that is totally natural as well, especially as we age.