Hello, Mikhail! You're comparing two things in your sentence, so you could say, "It's much/a lot simpler (comparative form of "simple") to do a thing than to talk that long." If you want to use "so," an adverb, use it with the base form of an adjective like "simple." But then, you couldn't compare two things in this case. Hence, you could only say, "It's so simple to do a thing" or "Doing a thing is so simple." :)

“It is much simpler to do something than talk about it for that long.” (“That long” implies a connection to a previous sentence) E.g...My teacher spent an hour explaining the task. It’s much simpler to demonstrate the task than to talk about it for that long.”