You can buy an old school paper book. A good one will come with or have downloadable content that goes along with each chapter. You can go at your pace and books are usually very thorough in explaining things because you can't really ask anyone a question. If you know how to do a chapter you could just skim it to make sure you don't miss anything, but not waste a lot of time on it. Plus books are cheap. You won't get a certificate for completing the book, but a certificate will only help you get your first job. After that having had a CAD job will be weighted much more than your certificate will in other job interviews.
Another good way to learn is to read these forums or other cad forums for say 30 minutes a day. You will learn all kinds of things that you didn't even know existed, but you could use daily in your work. You won't learn how to set up very many variables or what they do during an online class or even a associates degree level program. You'll learn a ton of that stuff on the forums however.
Edit: I also recommend Paul Aubin's books that are mentioned in the link. I went through his Autocad Architecture book a couple years ago to learn that vertical software. Very easy to follow and taught you things you needed to learn not fluff stuff that you will never use just to add pages and make the book seem big.
Nick DiPietro
Cad Manager/Monkey