What Causes Headache, Chest Tightness, Dry Cough, Dyspnea And Muscle Fatigue After Exercise?

Question: I am a 66 year old white male, non-smoker, minimal drinker, who is normally very physically active. Most significantly, when feeling healthy I bicycle recreationally 100-150 miles per week. I have noticed a gradual, mild loss of performance over the past 5 years or so as would be expected at my age, but over the past 3-4 months I have experienced an accelerated, fairly significant exercise intolerance at the high end (e.g., climbing longer, steeper hills). Symptoms with normal activity include fairly constant mild headache, a mild sense of chest tightness, mild mostly dry cough, and mild upper airway congestion. Symptoms with more intense exercise include moderate dyspnea, more rapid increase in muscle fatigue and decreased stamina compared to the past, and increased general fatigue in the 12-24 hours after exercise. While I can still bicycle at a moderate level for my age, this problem has noticeably affected the distance and speed at which I can ride and definitely reduced my enjoyment of the sport.
Other pertinent history: I have a history of 2 episodes of atrial fibrillation 30 and 20 years ago, with no recurrence. I have long-standing mild first-degree heart block on EKG. I have borderline essential hypertension, currently untreated. My resting HR is 60-66. 5 years ago my HR with maximal effort on the bicycle was typically 170-175; now it is 160-14, and the symptoms I described above become noticeable as my HR rises to around 150. My cholesterol, LDL, and HDL have always been well within normal limits.
Recent workup include CXR, blood chemistries, blood counts, Lyme screen, inflammatory screens, echocardiogram, and nuclear stress test; all have been reported as normal.
I would appreciate any thoughts you might have. Thank you.

Thank you Dr. Bhavsar; I will discuss your suggestions with my primary care physician. If my problem is bronchitis, do you think there is any chance that there is a bacterial component that should be treated? My CBC was normal 2 weeks ago and I have not noticed any significant fever, but I do feel that my upper respiratory symptoms have become a bit worse in the past week or two.
Thanks.