or is it just one of the many age-related things this 42 year old seems to be putting up with more and more?
Who knows? But 42, is a little young to be sensing any kind of big slide. I'm 54 and continue to maintain >90% of my aerobic capacity. Only recently have I come to terms with the decline of some anaerobic capacity.
Maybe someone can find a "study." But I'm pretty sure that the really noticeable fall-off in exercise capacity usually starts in the 50s, and becomes more severe in the 60s.....