Panning takes some practice. Faster moving, closer subjects are harder than slower moving, more distant subjects. Large objects moving at uniform speed are easier than small objects moving erratically.
Find a safe spot to sit, at the side of a highway, practice on the cars going past. If you can find a field by the highway, or the edge of a gas station apron, you can adjust how close you are to the road and see the effect of distance. You could also visit the local skate park and pan while shooting bicycle riders and skateboarders. Speed is mostly a factor if your subject appears and disappears quickly -- by that, I mean your subject pops out of some trees, travels, say 60 feet, and disappears behind more trees. Shooting airplanes a few hundred feet away, while they are travelling just below the speed of sound is not that hard because you can see them coming.
The larger your subject is in the viewfinder, the harder it will be to track. Also the heavier the lens, the more work is involved. It will come with practice.
Night will be interesting. I hope the track is well lit. Drivers will probably object to being blinded by flash. If you are panning to get a sharp car, and a blurred to streaks background, you will be shooting at a relatively low shutter speed anyway, even during the day. Night may just mean increasing ISO and opening up your lens. Some of the bigger zoom lenses have an image stabilizer with a panning mode, check your lens' manual.
Your 55-300 is probably lighter than your 70-200. I would just hand hold either of those to pan, but I would usually only be doing it for a couple of hours, not for 12 solid hours. It would probably be OK for 2 minutes followed by a rest of 8 minutes, or whatever, if the cars appear, then go off and run the rest of a long track. If it is a regular road coarse with left and right turns, finding a safe spot at a turn would work better than the middle of a straight section. Cars slow down a lot for sharp turns, but accelerate quickly when the straight section appears. They often bunch together in the turns too.
Hope you have a great day. Remember to apply sunscreen.