Stability: Keep paddling–you’re much more stable with the paddle in the water. Keep your hips loose and free from your upper body. Depending on the kayak (shorter, with “rails” on the hull), it can be steered by the tilt of the kayak, so it can be necessary to keep it pretty flat to go straight.
Speed: The power comes from rotating your torso, not your arms. Think about keeping your arms flexed and just rotating and using your obliques to power the stroke. As long as you paddle with close to the same power and take about the same stroke lengths on both sides, you should be solid. The power/speed isn’t helped much by a long follow through. Keep the strokes shorter and the paddle should come out just behind your torso.
Have fun and good luck!
Any chance you can get in a body of water and try the kayak in advance? Even 5 minutes of launching, exiting, and turning would be a big help.
I have a cheap wide short kayak that is very stable, easy to turn, and sloooooooow, however I used a 17’ expensive sea kayak that turns like a semi and is so narrow that it can be tippy, but it is made for speed.
The learning curve in the former is almost zero, but in the latter it is much harder.
Hopefully your friend can get you fitted adjusting the peddles and making sure the thigh bolsters are tight. Also make sure the PFD is comfortable and gives you proper range of motion.
As for the actual race it is nice the kayak is last so you can go all out vs. being first – expect about 30 minutes assuming the current isn’t too fast.
Make sure everything you bring on the kayak is waterproof!!! You never know when you will tip.
The other poster had good advice on technique, which is not my forte.