If I have a 2004 Honda Civic EX that I purchased new, is 68k miles considered
 

relatively low? I have used this car mostly on highway miles. Is this pretty good for it's year?

I've always heard about 15k a year. I also have a 2004 civic ex, but mine only has around 40,000 miles on it. But yours shouldn't be too far off the mark to raise any flags or anything.