It sounds like we don't actually have evidence that the GPs claim was true - that they made it up.

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.