Odds of Getting In
Sure, it seems like this should be the easiest thing in the world to start. Just put up a couple of Craigslist ads and some flyers at the local Parks Department office and you're all set, right? Not even remotely.
As we've said before, people want to know you have experience leading humans out into the unknown. They want to know that the person leading them out into the jungles has references and people who trust them to get the job done. This is where travel companies and outdoors clubs come in.
If they think you have what it takes, they'll send you to the right classes, assist your growth as a leader, and try you out on some of their less-dangerous trips. Unlike some dead-end office jobs, there are guaranteed ways to move through the ranks, provided you can handle the challenge.
Again, and we can't stress this enough: to really make a career out of this, you've already got to be fairly experienced with travel and nature. If you haven't been camping, biking, or sailing since birth, getting started on this career is going to be an uphill battle. So if you're looking for that experience, start earning those merit badges now.
Or you could just head on off to the woods and learn the hard way by living out there alone for a while. (Just kidding, you should absolutely not do this under any circumstances. If you really want to start going outdoors, go with someone who's already been a few times.)