Shoebacca wrote:
Mechanically: This has a lot to do with your biomechanics, acute injuries, and recovery. Runners with decent biomechanics who take time to heal after injury and ramp up their training again in responsible ways should do little longterm damage to themselves and mostly enjoy the benefits.
Cardiovascularly: Excessive training beyond what is aerobic or when dehydrated could cause your heart to grow, harden, or show damage in ways that could be permanent. Fortunately as a high mileage runner you're doing mostly aerobic running, so the stress on your heart shouldn't be too much as long as you are properly fueled and rested.
Chemically: High levels of training could lower testosterone, iron, or throw things like thyroid or endocrine systems out of balance. Most of this should be fixable with good nutrition and responsible training. Time off should help a lot of that come back quickly if you watch your nutrition.
Basically running is healthy if you do it right. If you overtrain or push anything beyond a boundary you run a risk of doing something that could take much longer to recover from. Then if you keep cumulatively compounding the stress on your body when already beyond what is healthy you enter the territory where a bone fracture could harden into a permanent limp or gait change, your strained heart makes a longterm adaptation, or you become dependent on a prescription for months to years because of some chemical shortage/excess.
I see a lot of older guys running with a sort of limp that has only gotten worse as they've aged. They just never found a way to change a biomechanic or couldn't bring themselves to take serious time off during a normal injury, so they never healed when the repetitive or acute stress happened and now they're defined by it. You don't have to become that guy if you take your health and recovery seriously. Soft tissue heals more quickly and so as we age it's easy to handle the aerobic aspect of training often to the detriment of bone or tendon. And then you have people who believe "no pain, no gain," and are basically setting themselves up for a health disaster when they're old. Just be smart about it and train within yourself.