There could be so many reasons why you feel this way. You can even consider on a hormonal level, depending on your age, gender, inherited health etc - we aren't designed to feel 'ok' all the time, because we aren't machines. Have you had regular down weeks and/or rest days scheduled in where you take a break from running or do less running altogether? Recovery is a part of your training program – it is just as important as the training!
You’re not a machine. You’re human, and you have to respect the body. Give it the time to strengthen and recover. Gains are made when we rest and repair.
If you feel as though your body needs a day off from exercising, then don’t be afraid to take one. Your mind and muscles will thank you for giving them the rest they need before starting up again with another training cycle. In fact, having a down week every 4-6 weeks is super important to ensure the body absorbs the work you’ve put in, and decreases the risk of injury. Time it up with your key races if you can, working with your coach and team commitments if you have one.
I also wonder if you have been fuelling enough for your training load - late-onset fatigue takes a while to set in after months of under-fuelling if this is a factor in your case?
energy intake has to equal energy output.
The biggest mistake long-distance runners make is not realizing the high level of calories they need to balance the energy equation. Most long-distance runners have low energy availability. This doesn’t just affect the body in its appearance.
It affects every single system in the body. Here are a few examples:
The gastrointestinal tract, causing slow transit time
Hormonal system
Psychologically, with interrupted sleep patterns
Neurologically, it reduces coordination and strength
Changes the metabolism, so instead more fat is stored in the liver
It affects bone mineral density
Increases risk of injury
A healthy balanced diet can be obtained from a diet of fresh fruits and vegetables, carbohydrates, alongside healthy and diverse fat and protein sources. We don’t actually need to spend gazillions of dollars on fancy supplements.
So, low energy input equals low energy output. I’m a big fan of eating before training in the morning. It doesn’t have to be a full meal, but working out fasted isn’t helping me achieve my goals of high-quality workouts and speed, strength, and endurance gains from these workouts.
I wrote a whole article on how to have more energy for running which covers all these points. Click to give it a read if you're interested:
https://larahamilton.com/more-energy-for-running/