By default, your Mac is not sharing any data. You need to do someting to open the Mac up for sharing. Since you didn't do that, you should be fine.
The likelyhood of your Mac getting hacked is very very low.
There are no known viruses, and only a handful of (mostly proof of concept) trojans.
Most security issues on the Mac are connected with Internet (Safari, Java). That's not a route a known person to you could esily take.
-t