People talk about "lung capacity" but for most people it really is not a limitation. It can be for people with lung damage or a lung disease.

The major limitation is delivering oxygen to you muscles. The oxygen delivered is influenced by several things. The first is the stroke volume of your heart. As people train, their ventricles enlarge. That means their hearts can pump more blood per beat. People like this often have very low resting heart rates. I recently flipped out my physician because I was having an EKG and my heart rate was in the low 40s. Of course, your max heart rate is another variable but I don't think that training has much effect on this number. The next thing that is important is the concentration of hemoglobin in the blood. The more hemoglobin you have, the more oxygen your blood will carry per unit volume. That delivers more oxygen to your muscles which means more power. Training will increase this. My hemoglobin levels are decent but nothing remarkable. Of course, if one takes EPO, that will increase the hemoglobin concentrations in the blood.

The thing is that at sea level, the blood leaves the capillary beds of the lungs at something like 99+% oxygen saturation. Healthy people tend to have more lung capacity than they can use. They are actually limited by their heart's ability to pump enough blood and their bloods capacity to absorb oxygen.

But when one is working hard, one gets the feeling of panting and not being able to "catch one's breath"? So what is this? When oxygen gets to be limiting to the muscles, the muscles switch from aerobic metabilism to generate energy to anaerobic metabolism. The thing is that anaerobic metabolism generates about 6 ATP per CO2 molecule generated as opposed to 1 ATP per CO2 molecule generated (if I remember my metabolic equations properly but at anyrate the difference is very large). Having high concentrations of CO2 in the blood stimulates the brain to tell you to pant. I suppose that is so you can blow out the CO2 from your blood preventing the blood from becoming too acidic. A small shift in blood pH is a very bad thing.

So what seems to be happening to you is that your aerobic capacity has declined some causing you to shift to anaerobic respiration sooner which results in panting. I would wager that you lungs are probably fine. It could be that there is a problem with your heart. It could also be that you are just having more difficulty maintaining your level of fitness and it has declined.