Delayed Onset Muscle Soreness (DOMS): This condition is characterized by muscle stiffness and pain after intense exercise, typically peaking 24-48 hours post-workout. Given the patient's intense workout routine and the timing of her symptoms, DOMS is the most plausible explanation.

Overtraining Syndrome: Intense and frequent exercise without adequate recovery time can lead to chronic fatigue, decreased performance, and muscle soreness. The patient's intense workout routine and potential inadequate nutrition (as hinted by her statement about not eating as much as usual) could contribute to this condition.
Nutritional Deficiencies: Inadequate intake of essential nutrients, such as protein, iron, or vitamin D, can lead to fatigue and muscle weakness. The patient's comment about possibly not eating enough suggests that nutritional deficiencies could be a contributing factor.
Dehydration: Inadequate fluid intake, especially during and after intense exercise, can cause fatigue, muscle cramps, and soreness. Although not directly mentioned, dehydration is a common issue in individuals who exercise intensely.

Cardiac Conditions (e.g., Coronary Artery Disease, Heart Failure): While the patient denies chest pain and shortness of breath, cardiac issues can sometimes present atypically, especially in older adults. It's crucial to consider these conditions, even if the patient's symptoms seem more musculoskeletal in nature.
Electrolyte Imbalances: Severe imbalances in electrolytes like potassium, sodium, or calcium can lead to muscle weakness, fatigue, and potentially life-threatening arrhythmias. Although less likely, these conditions require prompt identification and treatment.

Polymyalgia Rheumatica (PMR): This inflammatory condition causes muscle pain and stiffness, typically in the shoulders and hips, and is more common in older adults. While the patient's symptoms are somewhat consistent with PMR, the direct association with exercise makes this diagnosis less likely.
Myopathies (e.g., Inflammatory Myopathies, Muscular Dystrophies): These are a group of muscle diseases that can cause progressive muscle weakness and fatigue. Given the patient's age and the exercise-induced nature of her symptoms, myopathies are less likely but should be considered if other diagnoses are ruled out and symptoms persist or worsen.