After becoming a lawyer and practicing for a few years, I decided I need a Ph.D. in psychology to do the career I really want. So I went back last year to take a semester of undergraduate psych classes before applying for doctoral programs.
The big benefit of having already had a “real job” was that school seemed incredibly easy by comparison. I often was given only a few hours to write a 5-10 page legal memoranda, so being given a few weeks to write a 5-10 page research paper was laughable to me. I was able to spend very little effort on the four classes I took, and I got the top grades in all of them. My professors also generally liked me very much, because my “minimal effort” usually placed me amongst the more ambitious students in the class. I felt like I had things to contribute to class discussion because of my real-world experiences.
However, the downside of this was that I was relatively lonely during my time back in school. I had absolutely nothing in common with the undergraduates (between age 20-22) in my classes.
This is not to slam the undergraduates. I was just like them when I was an undergrad myself. I thought carrying 15-18 hours was a lot harder then, and I did not participate in class discussion much. But, there is a lot of learning that happens in the five years after a person graduates from college. If you want to make friends, bite back the comments you will be tempted to make when you hear your classmates complaining about how hard they have it.
Also, when I was an undergraduate, we made jokes about non-traditional students. My friends and I hated how they tended to “monopolize” the class discussion to “talk about themselves.” When I caught myself speaking of my real-world experience in classes upon my return, it was all I could do not to laugh. I was sure that I probably sounded self-centered and boring to a majority of the students. So I tried to make sure that when I did speak, it seemed very relevant, and that it was a point no one else had made. I do think this is a point to watch yourself on, particularly if you want to fit in with classmates.
I went to a large state university with ~20k undergrads (University of Iowa), so my experience is roughly analogous. Best of luck!