Vitamin C is important, but not so much for the warding off of respiratory viral infection. The main benefit that it would confer hard runners is that it is key to the absorbtion of iron.
The main reason we get colds and flus in the winter is because we tend to become vitamin D deficient in winter....we don't get enough from the sun. Personally, i take a 1000mg capsule of Cod Liver Oil every day to mitigate this. You can also just get Vitamin D supplement pills, but then you don't get the other benefits of fish oil.
Vitamin D's role in the body is only just being discovered. As well as helping us absorb calcium (important for bone strength), it is a key aid to our immune system. It helps in the prevention of Cardio-Vascular disease, improves the elasticity of blood vessels, is vital to the way our body deals with sugars/carbs (D deficiency is associated with diabetes and obesity)...there's much more.
Fell runners who spend a good deal of time in the sun during the summer (without sunscreen) will likely have decent D levels. In the winter months, it's essential to supplement through dietry sources.