Six miles a day on trails sounds like it’s a great exercise program. Walking is not nearly as punishing on the hips and knees as running, and hiking on trails means varied terrain to reduce chances of repetitive-motion injuries. I am not a health professional but it doesn’t sound like anything you are doing could have long-term negative consequences.
The guy in that article was doing up to 30 miles a day. It got a little obsessive.
30 miles per day. Whew. I would rather take several time-outs and enjoy my surroundings. YMMV.
I walk a similar mileage but on fairly flat terrain. I would not call any of what I do hiking.
As I got older (70 now) I discovered that I would get knee pain. I found that pain would resolve itself with exercise, so I do those exercises regularly (leg lifts mostly) and I never get pain. The same is true for my hips (I have two resurfaced hips). Basically my takeaway is that walking is not enough to keep you walking; you need to supplement it with “physical therapy” exercises. Best is going to a physical therapist, but you aren’t likely to do that till something goes wrong, so check out Youtube for preventative measures.