I recently shot a friend of mine who is a member of the GB triathlon squad. We headed out to Richmond Park on a cold August morning, and despite nearly freezing the death at first, the sun came through for an hour or so at about 7am.
There are a couple of techniques that you ned to nail for these kinds of shots.
The first is mixing ambient light and flash with a moving subject. Because your camera will only sync with the flash at 1/250th, you need to be careful not to get motion blur from the low (for action photography) shutter speed.
The second is panning. When you actually want to have motion blur, rto capture the sense of speed, you need to be sure to get just the right amount. Ideally you want the background blurred and the subject remaining reasonably sharp. When shooting close up with a wide angle lens, this can be particularly challenging. The solution is lots of practice and re-shooting again and again until you get it right.