You're in a dream position.
That being said, there's much work to do. This job will look great on your resume in the future, but you need to hold it for a few years to build credibility.
Ignore the "Install Gentoo" comments. While you can research open source options to fill software requests and work to move from proprietary software, jumping straight off a Windows environment right away is a mistake. Unless you're a graybeard (and you're clearly not) you'll be in over your head trying to do custom Linux installs that will require tons of work to reach parity, and you'll end up making employees uncomfortable.
Instead, focus on automating the bulk of the tedious tasks. Write scripts to fix common problems users have. Write scripts to automate new employee set-up. Write scripts to automate employee terminations. Make it so that you can pull a string and the network will continue working.
Spend your time learning new certifications. Diversification is good. Do some courses on security and penetration testing. How secure is your network? How's your backup plan? How much of your network can utterly fail and how long will you be bent over? Your nightmare scenario is an utter failure of the network one day leading to your public firing or your organization being publicly shamed for the failure and taking a hit from it. You're also the sole employee in IT, which means you will take all the blame and also spend multiple sleepless days trying to string a fix together. You can never truly solve security and backup management, but you can do a lot to make sure you can avoid 90% of all nightmare-class scenarios and easily repair 90% of the ones that might actually get through.
Aside from that, continue to find new IT skillsets that are in demand. Browse job sites and see what other companies want experience with. A lot of it will, unfortunately, probably be cloud or virtualization, but you should still get familiar with these just so you can pad a resume at the very least. Look for trends and be sure to spend a little time each day learning them.
Final note: while I encourage you to optimize your network, automate the bulk of the work, and spend most of your time learning, you should never, ever, ever, ever, ever, ever, ever, EVER forget to show your face to the other employees to remind them you exist. The bosses especially. Walk through the office to get your coffee. Take trips through populated areas. Leave to grab lunch. Stop and say hi to people. When they have problems, always listen and try to find a way to accomodate their needs without ruining your network. People like that. Your employers will like that. Becoming a hermit who virtualizes their network and does all support remotely is an amazing way to make everyone resent you and have nothing nice to say about you, but you'll still take the blame when something inevitably goes wrong, even if it's just their e-mail is slow to load.
Do not make the mistake of disappearing entirely. You can screw around all day as long as people see you rushing between offices now and then. They don't need to know why. You don't need to tell anyone what you're doing exactly. Just always prioritize users and get their stuff done in a reasonable amount of time and be sure they see you being active.
Your goal is to build an incredible resume with a great title and experience/certificates in all the major areas employers are looking for right now. In a couple of years you can apply to be the IT Director of a slightly larger firm and go from there. Or, if you're comfortable there, you can always push to have someone under you to handle menial tasks and help you in case of emergencies. If the organization expands this will become necessary. You may be able to use job offers to barter for a better salary and more benefits, too.
Do it right and within a few months the network will run itself, you'll have decent security in place, you'll have emergency plans drawn-up, your entire network should be fully documented with all the information anyone would ever need to manage it, and you should have an amazing resume to pass around. Nobody at any company will ever know you skipped ahead and don't have the same level of experience as others with your title, and they never need to know.
Good luck anon, and godspeed. Let me know if you have any further questions.