High carbs=inflammation plus plant-derived toxins. Just like smokers exposed to oxidized plant toxins. Same with seed oils (omega 6 especially), with the kicker that they integrate and embed in all the tissues to speed up aging and undermine cellullar integrity. Subject such tissues to sun exposure and you have cross-linking and telomere damage. Also collagen is dependent on proper animal-based proteins, as is bone density and integrity. These are all structural components that affect lines, wrinkles, scarring, moles, age spots, etc..
The other front is hormones, which are undermined by diet low in animal fats, whole animal protein and animal-based bioavailable micronutrients, including antioxidants. Hypothyroid, anemia, cortisol stress, low sex hormones, all accompany plant-skewed diets, especially in conjunction with overtraining and lack of proper strength/resistance training.
In short, the poor diet and redundant stress that runners subject themselves to are all relevant to accelerating aging on many levels. The healthy elements of the activity dwindle after 15-20 miles/week, especially without a proper balance of resistance training and whole-animal foods such as beef, eggs, fish, butter, cheese and raw dairy.