A few things I've learned from shooting Cross Country that might help you:
1) Use a small aperture (but not too small), such as f/2.8 or f/3.5. f/4 is good too. This separates the competitors front he background, which is really important, as meet backgrounds are often very bright, colorful, and distracting from your subjects.
Agreed. Play around with a depth of field calculator and see what the DOF is @ 200mm from 50 yards away... and then at 20, then at 10.... etc. Or... just start shooting at 50 and don't stop until the subject passes you. Don't be afraid to use the buffer to your advantage until you are more comfortable with photography.
2) Prefocus on a specific spot. A tripod or monopod will help you with this. This is the best method to freezing action that I've found, as it guarantees sharp shots when the runners are in that zone. Use your 1DX to focus on one spot on the track, and when the runners are roughly 5 feet away from that spot, start mashing the shutter button.
I disagree. I used to do this with my fuji finepix and my sony point and shoot... but with both a 1Dx and a 5d mkiii, both should easily be able to keep the subject in focus... and if you are dealing with shallow depth of fields... you are going to only have one moment to get the shot...
3) Use at least 1/2000th sec shutter speed. Self explanatory.
Agreed. If you want to see a little blur... go with 1/200... but it does depend on the speed of your subject.
4) Put your camera in AI servo mode, it saves time on AF
5) I hope this goes without saying, but don't use the 5D III as your main body. I wouldn't use it at all for action if you can avoid it.
Well... compared to the 1dx... sure... but the mkiii is excellent. The mkii... sure... but the mkiii...