You probably have an email list of at least a hundred people interested in hiking, but keep building on it. Keep them posted with new features. Keep reminding them to forward to others. The site can be utilized to gather users, but few are just going to stumble on the site; someone they know/trust will need to make the recommendation. There should be no confusion that you want people to sign up and not just browse the site.
There should be current users, hiking groups and events displayed on the home page. As you build up a larger user base, you can use this space to reward users and groups who are promoting the site. Have two distinct paths: those organizing hikes and those looking for hikes. Either way, encourge them to register.
Take advantage of any pay per click you have and work on identifying the keywords.
Blog if you can or provide a place for a few people you know that would be interested in teaming up on your site. Blog content can take advantage of SEO as well. A large amount of changing content with the same theme is appealing to Google.
Build links. Know someone with an old site and lots of links? Build this up gradually, but continuously. Google won't like it if you try and outsmart them.
Offer some advertising space to someone who has a small product niche or is a local hiking guide or a non-profit summer camp.
If you're more of a programmer, create the tools to allow others to help with the content.