I have been researching vit D for years. I generally get mine by taking 1000mg of Codliver Oil daily.... as the oil also confers other benefits.
The main (VitD) benefit seems to be (and this is just my experience...shared by others) that my immune system is good. I can share a bed with a snotty, sweaty, sneezing, coughing and otherwise lurgy ridden MrsMoors without suffering much more than a sniffle the following week.
There are cardio-vascular benefits, which are of interest to runners... chief among which is improved elasticity of blood vessels.

The other thing is that I make sure that if the sun is out...even in Feb, I will run with plenty of skin exposed.