I have been pondering this for some time. I'm no athlete and I was crap at sport at school. I just don't seem to get fitter or faster or more able past a certain point. I can now run a 5km distance but can never get quicker than 33mins. My average time is 35mins. I've tried interval training, i've tried running slower, running faster. I'm not particularly interested in getting quicker but I have tried over many decades and it just doesn't happen. I'm perfectly happy at around that pace but now that I'm about to get to 50, it does make me wonder whether I just had a natural 'limit' to my fitness.
It's the same with bike riding and swimming. I can do a certain amount but just no faster and no longer. I recently went on holiday for 2 weeks and did nothing more than long walks and when I came back and went for a short, slow run, my muscles hurt like I had never done a run before. The same happened when i hopped back on the bike. It's as though my body is so crap at sport, it has to relearn all the time.
Just interested whether others feel the same way. As mentioned, no interest in becoming an athlete at such but enjoy keeping fit for health reasons!