It sounds to me like you're simply pushing too hard and running out of gas. No matter how fit a cyclist is, there's always an intensity they can ride that will exhaust them in an hour. Better to ride at a pace that is sustainable for the entire ride.
The low-card diet is a possible contributor to the issue. If you have a feeling of quickly become very tired, like hitting a wall with the effort, you my be bonking (running out of stored carbs).
While it's possible for some people to adapt to riding at higher intensity while fueling from fat, generally there's a point where the body simply cannot convert fat quickly enough to keep up with demand. Normally, an hour ride is short enough that you shouldn't need to eat while you ride. But if you are restricting carbs, then you likely have low glycogen stores (stored carbs). Glycogen is quickly and easily used to fuel muscles. In fact much of it is stored in the muscles themselves.
Again, normally you would have several hours worth of glycogen and wouldn't have to eat while riding. Though there are still reasons to do so.
I have no idea how to approach this while trying to maintain the effect of a low carb diet. Except to say that if you are consuming carbs but then immediately burning them, I would assume it wouldn't upset the low-carb diet too much. But I'm sure there's people on the forum who know about this. You certainly aren't the first person to do endurance exercise while on a low carb diet.
I know with myself, when I'm generally calorie restricting (though not low carb) to take off weight, even though I fuel rides, I simply cannot ride at as high an effort. If I've been over eating a bit, say around the holidays or similar excuse, I can ride much stronger for longer and feel great. This is the effect from "carbo loading."
Not eating enough for your ride can tend to encourage over eating later in the day after the ride. If your fueling the ride properly, you shouldn't feel hungry or have a huge appetite when you're done riding. That can be a sign of not fueling the ride properly.
Of course, not drinking enough can be an issue too. But unless you're riding in a hot and humid environment, I doubt you're getting dehydrated in an hour. And you mention drinking Gatorade.
Keep in mind that there are advantages to doing a lot of riding at a low pace. They call this zone 2. You should still be able to talk while riding at this pace. Also, at age 65 (I'm 63), riding everyday may be too ambitious. Like any exercise, the gains come during the recovery after the effort. If you don't give your body enough time, nutrition, and sleep, it can't recover properly. Going out and trying to hammer every ride is generally not a good way to build up fitness.