I'm with accountantpete on this, Scots Lass. University courses are increasingly aimed at obtaining a qualification and/or a job, so if you are looking at learning for the fun of learning you may find the process disappointing.
If your purpose is to study at that level for the first time, to prove to yourself that you can do it, then by all means sign up for it. Ditto if you find that you need the discipline of fixed hours and deadlines, which many of us do.
If you are looking for something to keep your brain active, there are many options apart from formal study. I retired last summer from my full time job, but stop my head from turning to mush by maintaining a presence in a couple of ventures which were sidelines; I've also set up a small charity; and I mentor two young graduates. Exams occupied every summer for me from the age of 10 to 23, so I would find very limited value now in absorbing other people's heavily recycled information, compared with doing something off my own bat which makes use of whatever skills and contacts have found their way into my head over the last 35 years.