If it's not a medical problem, you're just riding too much and recovering too little. You wouldn't think so from reading Bike Forums, but that's pretty common, even if nobody over 50 will admit to it. Don't know if you're doing this, but dieting and high mileage also don't go together very well. You need to replenish calories, and you need to eat protein to allow the muscles to repair themselves.
It helps to do high mileage if you do it very light on the pedals. Avoid mashing, and use easy gears to go up hills.