Making a living from writing is not easy. From what I have read and heard from people who write much better than I do, making a living in the world of writing is downright tough. Travel writing is probably no exception. Phrases like "nearly un-doable" and "like beating your head against your desk repeatedly" or some variation of them are oft heard by travel writers discussing how to get started in their trade.

Is it that these writers are afraid someone will encroach on their assignments? Or is it really that tough to make it as a writer?

Writer Rolf Potts, famous for his website Vagabonding, dispenses some advice for would-be Pico Iyers and Bill Brysons:

[O]nly get into travel journalism if you really love to travel and write. If you think it's a good pretext for getting to travel, think again: you can travel just as much by saving up money from another, better-paying job, and just taking off to go vagabonding.
So only pursue travel writing because you love to write as well...and even if your travels don't lead to a full-time career, they are a reward in and of themselves.

If that doesn't do it for you, try this advice from Karl Taro Greenfeld. It comes from the other end of the spectrum as far as outlook, but says basically the same thing as Potts' statement:

Just travel. Have a blast. Get loaded. Meet girls, or boys. Just lose it. And if you still feel like writing despite all the other, better things there are to do in life, then I guess you have no choice.

Of course, cutting your teeth with a blog or even some low-paying gigs can help you decide if travel writing is a path you'd like to attempt. (A lecture by Potts can be found here. It was an hour long, so I chose not to embed it in this post)