I'm not sure what a road and fell race is, but sports in general usually requires fast action. The easier way to do that is to use the sport mode - it'll set a fairly high shutter speed and let the aperture open as wide as it can for the lens you have and the zoom you are using. It'll also set it to focus continuously (AF-C) on one spot (by default the exact middle, but you can move it around) and disable the flash. This is a good starting point.
Next step would be to switch to "S" mode, and set a shutter speed that is fast enough to freeze the action (assuming that's what you want - but it's a good place to start). If you can get away with a lower shutter speed, the aperture can float a bit more, which is especially useful if you're taking pictures in darker situations (indoors, evening, etc.) or want blurry backgrounds. You'll want to set the autofocus mode (as you're holding the camera, hold the button on the AF/MF switch below the left side of the lens, and rotate the thumbwheel) to AF-C, continuous focus mode, then rotate the other wheel to set it to focus on one spot. Let go of the button, then use the thumbwheel to adjust your shutter speed.
Try starting with 500 (1/500th of a second), but you'll know pretty quickly if your subject is blurry. If it is, a faster shutter speed is required (larger number). If not, try reducing the shutter speed (smaller number). Don't be afraid to zoom in on the picture to see if it's sharp, but note it won't be sharp if you zoom in too far. It's free to take more pictures to try different settings!
It's really a trial and error process, and that's how you learn. One other general tip - usually you want to try to zoom in on the subject as much as possible, but note that as you zoom, with most lenses, you need a larger aperture to keep the same exposure, so if it starts getting too dark for the shutter speed you need, you might be better off to zoom out a bit, get the shutter speed you need, then crop the picture later.
I'd also set it for CL (using the lower part of the left wheel on top) - continuous shooting, low speed. High speed sounds great until you end up with 30 pictures of almost exactly the same thing to sort through - it's useful for things that are going REALLY fast and you're trying to catch it, but not so useful in most situations. I'll usually fire off between one and three shots of a sports subject that's moving, like a person.