If you had follow-up Xrays and the lungs were clear then your pneumonia should not be the issue. I would say that if you are not as active now as you used to be it could just be that you are not at the same level of fitness. We get busy and stuff, don't work out as much, as we get older. I also think I could hold my breath a lot longer when I was a kid than I can now. Maybe that is just a common thing as we mature and not really indicative of fitness.