Well if you want to organize, maintain and prevent malicious attacks, then I would assume Network Administration would be up your alley. Not only would you setup your organizations infrastructure, but I'm sure you would want to make sure that infrastructure is secure! ;D Note, I use the term Network Administration loosely here. If you wanted to be more specific, you could says System Administration and Network Administration (the former being and Admin of let's say a Windows Environment, adding users, setting up domains, etc. While the latter being an Admin of Routers, Switches, VPN and more network hardware). Certainly, many of the roles over lap and many employers prefer you know both, but just to be politically correct, they are two different roles.
Sorry if that confused you, but let's get back on track. Network administration seems to be up your alley, protecting from intruders. Now other aspects of security include Managerial roles (where the CISSP would benefit) where you understand security, but don't actually implement it. More or less write security policies that your organization would comply to. There are security researchers(mostly independent work, but some people get lucky and do it for a living) where you look for vulnerabilities and write exploits (for the good of course). You have penetration-tester(sometimes called ethical hackers) Where your services are used to actually attack an organization and provide a report where the weaknesses are. There are auditors (close to penetration testers, but not the same) Where they find ONLY vulnerabilities, but not actually attack them. (the debate as to which is better can be found elsewhere on the forum on the internet)
This is just the beginning, and there is a lot to learn. I hope this helps get you started. Good Luck!
eLearnSecurity Team Member.