It is not a vegan issue, it is an entire animal kingdom problem at the moment. B-12 deficiency is around the same percentage among vegans, vegetarians, and meat-eaters alike. We are naturally supposed to get B-12 produced from bacteria in the water we drink and from the bits of soil on the vegetables we grow. But because of modern agricultural practices that sometimes compromise the nutrients in the soil like B-12, humans and other animals end up deficient. If you are eating out and order some sort of animal product (meat, dairy,eggs, etc), you're basically guaranteed getting a B-12 supplement because the animals, especially those in factory farming, are not exposed to a healthy environment rich in essential nutrients. Many vegans and vegetarians can go years without supplementation via pills or fortified foods like almond milk, nutritional yeast, granola bars,etc because B-12 can circulate in the body for many years.
The consensus among traditional and plant-based doctors is to supplement or eat fortified food for BOTH meat-eaters and non meat-eaters. Unless you are living on an organic farm in the countryside where you are drinking stream water and growing your own food, it is likely that you could end up deficient regardless of your eating patterns.
To tie this to running, Eliud Kipchoge and many other prominent East African runners rarely eat meat. If they do, it is only for a social aspect maybe once every couple of months. Why? Because they get all the nutrients they need from a high carb, starch-based diet consisting of foods like rice, beans, ugali, collard greens, potatoes, bananas, fruit, and a whole assortment of vegetables.