Quicksand isn't nearly as deadly as the movies would have us believe. You can be killed in it, but if you behave sensibly then you shouldn't find any trouble getting out.
Quicksand is, essentially, a mix of sand and water. That means it looks like sand, but when you step on it you sink into it like water. However, by definition, it's denser than water, and that means two things: 1) you sink into it less quickly than you would if you stepped onto water 2) you can float in it, or swim - more easily than water in fact. The problem comes because quicksand is very sticky (viscous) and if you step in it, it can be hard to pull yourself out.
Here is a National Geographic article giving the science, and an explanation of how to get out of quicksand if you end up in it.
If you do step into quicksand, you'll only sink in a little deeper than your waist. "I would say there would be some pressure on the chest, but not enough to cause serious trouble." So how do you get out? "The way to do it is to wriggle your legs around. This creates a space between the legs and the quicksand through which water can flow down to dilate [loosen] the sand. You can get out using this technique, if you do it slowly and progressively."