I struggled with running at school, a combination of living in a household of several smokers and never actually being taught how to run were pretty major factors. I was also consistently the smallest in the class so with teachers bellowing encouragement like "you're not even trying" as I maxed and burnedout trying to keep up with children with legs several inches longer.
It was my mobile phone that taught me how to run and that slowing down is the key to not burning out within 30s.
Fortunately I was an active child, active teenager, motivated to sort out deficits like learning to swim and cycle in my late teens and have gone on to be active through adulthood.
Because my PE experience was so shit through the entirity of school, I encouraged activity for my DCs right from the beginning. Just as well as one is dyspraxic and autistic and has the odds stacked against him. The years of junior parkrun have paid off. While he's very average on parkrun's age grading, he is the fastest in his PE class despite being the smallest.
While running isn't the be-all and end-all of sport, if you can't run, it makes most school based sport difficult. If you don't do any sport out of school, that's creating a very negative foundation of sporting experience for life, and that's hard to turn around. I just accepted that "the girls in our family are bad at sport" and it was having to take up a physical recreation for DoE that turned it around... if I could learn to swim 25m, I could learn to ride the bike. If I want to walk up the hills, I have to do more walking- I'll walk instead of catching the bus. With hindsight, my couple hours of dancing per week didn't make me particularly fit as a lot of time was wasted on taking turns and waiting, but it did give strength and co-ordination and made going to aerobics, step and yoga easier in my 20s.
The standard of fitness and health in my youth groups has fallen noticeably in the last 5 years. Children too weak to use climbing walls and assault courses. They're the same children who 2 mins into a normal game are either beetroot red and pouring with sweat, puffing, or are dropping out at the sides of the game. The same children that are slow on walks and struggle to get up from sitting on the floor. In short they do not have a functional level of basic movement and fitness which is an awful foundation for life, and awful for their confidence.
Parents need to give their children opportunities to move. At a young age, play areas, walking, scooting are free/ cheap. In school, making time for things like the daily mile brings many benefits into the classrom. Fit children have an advantage for learning- their effective circulation feeds their brains with feel-good hormones and they can apply their mental energy better. There are ways of actually teaching PE rather than just bellowing to move. But ultimately in the early years, parents are the strongest influence on child activity and health.
Communities develop their own normal. The difference between the DC's primary school where most families can afford private swimming lessons and the schools from less affluent communities that were before/ after school swimming was stark. Both in the proportion of children in the different ability of swimming groups and by body mass. DS1 went from average in his primary school where most boys took football training seriously, to being one of the fastest at his much more diverse secondary- this is a teenager who loves nothing more than festerjng in a gaming chair in a darkened room if he can get away with it- he doesn't not live and breathe sport.
There will always be a small number of children with medical or developmental difficulties and that needs managing sensitively, but the majority of children should be able to run short distances, climb, walk at a functional pace and get up off the floor and there are too many children being failed by their parents and society's set-up.