I havn`t shot athletics but have done a bit of soccer,my recommendations would be and you will need to try these before hand,and listen to more experienced people here.
ISO You know the limit you will accept, you might need it if it is overcast.
Shutter speed 1/300+ depending on ISO and aperture and if you are hand holding.(the general rule is SS the same as focal length but I have used a lot lower) practice
Apterture Apteture Priority
Metering spot is what I use the most, centre weighted if the subject fill the frame.
Continuous low would be the way to go,if you shoot high you could have 90 shots (20 seconds to run the distance x 4.5) guess at time to run race.
You will need to try these setting an adjust on the day, also if you use raw it stops after 11 shots but the buffer is not full it will start again by releasing the shutter button and take 11 more shots.
(this is from DPReview.) hope you have time to practice.
Jack