I definitely wouldn't consider writing a book as an alternative career - most of the working writers I know don't even consider book-writing their main career. Everyone I know that's ever had any success with writing books also does something else - teaching, writing for newspapers/magazines/the web, research at a Tier I university. It is extremely rare that a writer can make a living on writing books.
With that said, if you love to write, then write. Write every day, as much as you can, and work on the craft. But if you do it with the expectation of profit, you'll probably be disappointed - do it because you love to do it, and you never will be. But remember that you're competing for publishing deals with people that have been trying to do this for their entire lives (I've know that I wanted to write since I was five), and the vast majority of people don't make it. It's not a viable career alternative even for most professional writers.