There are many factors that go into whether someone is healthy or not. Sometimes people have undiagnosed heart problems. I have a pet theory about those who are on strict vegan diets and then run a marathon. I think that pure vegetarianism puts a strain on our hearts, and also affects our muscle and tissue strength and replacement. Plus, many runners eat "nutrition bars" and special sports drinks..both often loaded with sugar and highly refined carbs.
Then there's the issue of genetics, a big one. Plus, sometimes these folks have hidden bad habits that no one knows about (bulimia, smoking...) you never know. And sometimes there's a lot of stress in their lives. Most secular folks are heavily burdened by the weight of living lives out of touch with their consciences.
For me, hitting middle age now...I've lost interest in doing much running. And I sure don't care very much about speed! So I run a little, walk a lot and try to focus on a well rounded diet ... not restricting an entire food group for example. And finally, I decided it was better to spend time praying every day rather than doing crunches, if I had to choose between the two due to time constraints.