There is a mitzvah to sit in the succah during the holiday. Is there a mitzvah to also build one? Well, if you don't build it, how can you sit in it, you may ask. How about if you go to a hotel for Succot, or have the gardener, the builder or the odd-job man build your succah for you?
There were great rabbis, including Rashi, who opined that building the succah is an additional mitzvah, but the view we follow today is that building is a preparation for the mitzvah (hechsher mitzvah) but not one in itself.
Even so, many say you should at least do some of the building yourself and begin your succah right after Yom Kippur to go from "strength to strength".
If you're too exhausted after the fast to get up a stepladder, start planning your succah or learning about how to build so as to move straight from the high of Yom Kippur to the next spiritual task.