Long extension cords will be needed to power your tools. High quality is important if you are going a long length.
Many do not realize that the long the cord, the more difficult for the power to reach the power tool. Saws and drills also draw a large amount of current and you will be putting them under a heavy load for this job.
I suggest using the shortest cord possible when using your circular saw and saw horses to cut the fence posts. This will save you from possible injury, reduce the problem of throwing a breaker switch, and reduce any chance or harming an expensive power tool. You really don’t have much choice with the drill because it will need to be powered at points far from the power source, but drilling requires less power than the circular saw.