I covered a 5K run about three weeks ago with the 70-300VR, it was 08.00hrs and the light was fantastic, slighty overcast, just right, however, after about two hours [1276 pictures] it had begun to get very heavy and I wished I had brought a monopod with me.
Pictures were fantastic with only about 15-20 dicarded, not the lens fault. used it at f7.1 for max sharpness and good DOF.
I used a friends 300mm f4 to cover boat racing, it is fantastic, however I was sitting on the pier with my elbows on my thighs and left wrist on my knee, it was fast and sharp and a pleasure to use with the boats comming towards me, however, I do not think that I would have wanted to hand hold it standing up for more than a few shots. this was at 13.00hrs in bright sunlight, so, I had my choice of aperture and shutter speeds to play with. fantastic, get it if you can afford it.
I use the 70-300 for birds, however at 300 you need to stalk to get close to them, or shoot raw and crop.
Also used a friends C------40D with the sigma 80-400mm, it feels solid, big, heavy, and will flare badly if you are shooting into the light. did I say big, cannot comment on the focus speed, only played with it for about half an hour, and was not familar enough with the auto focus speed of he C----- to say if it is fast, the 70-300 is faster on a Nikon,however, if you pay attention to the lighting conditions, it did a nice job.