The good news is, you can learn a lot on your own. The bad news is, it's pretty hard to improve if you don't have anyone to push you to improve and it's hard to know what works when you don't have anyone to test yourself with.

It's kind of like learning a language. You can read books, listen to tapes, and make flash cards, but if you don't have another person to talk to, it's all just theory that exercises your brain. Same with the martial arts. Also, without an experienced practitioner to correct you, you're inevitably going to do some things poorly without knowing it.

If you are pretty much set on training without a teacher/school, at least find someone you can work with regularly to test what you know. That said, if you have a least a basic MA background and know rudimentary body mechanics, you can still build good strength and reflexes with time and dedication on your own.