Good for you. I did the same and it led to a completely different life, some wonderful experiences, and getting married, so go for it.
I would recommend a bit of pre-reading, if poss. I had been out of education, and was studying political science for my MSc, so I was definitely behind the curve for the first semester with all the bright-eyed, bushy-tailed politics and economics graduates.
Also, employers will often offer study leave and time off during the week - if you've not already got this, it may be worth it.
On essays and coursework, don't spend too much time fretting about getting it perfect - crank them out quickly and move on. You will have a LOT of work to get through.
Enjoy yourself.
Oh, and pick a dissertation topic early so that you can work towards it and gather material and info as you go, otherwise you will be floundering around with 15 minutes before the topic submission deadline, and that's not a nice place.