Rather than digging deeply into the esoterica of current research on the human body, Tim Worstall suggests there’s a handy rough metric you can use to judge your own physical fitness in any given area that sounds helpful:
“Exercise Running Fitness Physical Activity – Credit to https://homethods.com/” by homethods is licensed under CC BY 2.0 
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If you can do the whatever it is in under twice the Olympic time for it then that’s just fine. You may, at that point, step off the treadmill and go get more dip.
Not that I have any expertise in such things as fitness — just the normal amount of forced labour true of anyone who went through public school. It still seems to me to be a good guide.
True, the accuracy of this varies depending upon the specific activity. Managing 100 metres in 20 seconds is not a huge call — only just managing it would have small children jeering perhaps. But a mile in 8 minutes, yes, that does require a certain level of fitness and one that’s also indicative of, well, being at a certain level of fitness. Not that I’m going to do anything so gauche as check this, but that sounds like about the fitness tests for middle aged men in the military (longer if it’s in full kit).
One recent Tour de France time trial was around the 45 km mark. Which they did in 45 to 49 minutes (again, from memory) and doing 45 km in 90 minutes is something the average club cyclist would do on Granny’s bike, with the basket in front. A professional cyclist would need to add a sheepdog to the basket to be that slow. But being able to crank out 45 km on a bike — in that hour and a half — is showing a level of fitness that I take to be just fine for the average couch lizard.
So too the mile swim. Olympic swimming is 1500m, in 14 minutes or so. So, a mile in half an hour? That looks quite testing but if I can get close to that (in my 60s) then I’m happy. I can swim a mile, which in itself is a reasonable level of fitness, but that time would, I think, qualify as being “fit enough”.
It’s possibly true that this guide is more accurate at the longer distances. For being able to even perform the longer distances is itself a guide to fitness and the time recorded is less of an issue. It would also be possible that personal experience is playing a part here — I’ve always been comparatively better at longer. Few fast twitch fibre. So the “Worstall’s” could well be “As applies to Worstall” rather than something more general.
I’ve always been bad at endurance sports, but I’ve done fairly well in sprint-style, “twitchy” sports where you need fast reactions rather than long, slow-burn exertion. Tim’s rule of thumb seems to be more useful for runners, swimmers, etc., than for badminton players or fencers.
