Advice for Photographing Runners (1D Mk2)

I have been photographing a number of Running events for a friends event business, mainly short 10K races with my 1D2N. My brief is usually to bag as many runners as possible at full length as these sell the best apparently.

I have a question about the techniques for this. I always set a single focus point as the upper most point in portrait orientation and I am automatically drawn to ensuring that focus point is on the runners face.

Doing this ensures the face is pin sharp and I get a very high keeper rate BUT in doing this I find that if I do a full length image the runner quite small in the frame, OR I have to cut off the limbs to achieve a bigger framed image and even then this leaves a lot of space above the runners head in the frame.

I guess my question is would I be better off letting the focus points drop down onto the runners body to getter a fuller framed image? I am a bit nervous about this as I like to use f4 on my 70-200f4L which would give me a fairly narrow DOF.

Any help with this would be appreciated.