Is this medical advice?
I would also assume it's obvious to most readers here that GP's guidance don't necessarily have to be the reasons for increased prevalence of colorectal cancer, but they would at the last help decrease chances of other health-related issues.
It's fine that walking and moving is healthy, but it's a motte and bailey to claim that sitting is a cause of colrectal cancer and retreat to the motte that walking and moving is more healthy in general. Such a retreat doesn't exactly lend credibility to the original claim; it subtracts from it.
If you're ok with it, I'd like to stay focused on the claim: That lack of moving/walking with long periods of sitting, being overweight, no fruits or vegetables and too much highly processed food/junk is the cause of increased colrectal cancer.
> ...but it's hard to live a better life for most as the above is just the norm now.