Your chances of success will always be greater when you follow a true passion. If you are massively into rock climbing then you should be making climbing films. The same goes for sailing, mountain biking or whatever. Make short films about the things that excite you as a traveller and adventurer. Get those films onto YouTube and promote them aggressively. Do profiles of the people you admire. Be bold in asking for access to people (travellers, explorers etc); they will generally agree if there is publicity to be had.
There are loads of great sites for film makers. Vimeo is a dynamic home for anyone planning to make a travel documentary or a short drama film. There are also loads of great networking sites like shootingpeople.org where you can market your skills as a film maker and meet other crew.
Everyone wants to be on camera – that’s why there’s a shortage of good sound recordists. Why not explore the options of working on sound? It is every bit as interesting as camera and may lead you to a foothold in the documentary world a lot faster than camera work. Movie makers and commercial directors are also always looking for dedicated and talented sound recordists.
It’s amazing how many people get their first breaks working as runners on low/no budget shoots. I started out working for free at TV-am as an intern and got my first break as a result. So don’t be precious about it; get your hands dirty on a variety of film shoots and get to know the camera crews. Opportunities may take a while but they will come if you are persistent and reliable.
Watch out for the programmes you most enjoy and make a note of the producers who make them. Track them down (it's easy via the internet) and email them to say how much you enjoy their work and to enclose a CV. Update them every three months on your activities and take them for a coffee if you can.