Hey Dan,
As Jim says, what constitutes "long" differs not just by the person by that person's current goals, fitness, injuries, etc.
The smart thing to do is build gradually from your current long distance, for you 10km, to whatever your goal is. Conventional wisdom as I'm aware of it is to be add no more than 10% per week to your long run/ride and give yourself recovery weeks where you realy cut back your time/distance.
My long runs are currently in the 1:30:00 to 1:45:00 area, and gradually building, but I'm training for Ironman and have run a couple of half marathons in the past four months. If I was building towards an Olympic distance tri then my long runs at this point in the season would likely be about an hour (for me that's about 10km).
As an aside, my coaches encourage our group to focus on time rather than distance for our long workouts. That allows me to run within the limits my body is placing on me at the time I'm running rather than force myself to go harder than I need to. For example, if I'm having a rotten day, instead of pushing too hard to finish a set distance I can end my run as soon as I hit 45 min or an hour and a half, whatever the time for the day is, rather than finish 8km or 14km. Conversely, if I'm having a great day I might go a little longer in the same time.
When I started training for my first half marathon I guessed that I could do it in 2 hours (based on previous 10km times), so in my training I gradually added time to my long runs until my longest one was 2 hours. I didn't cover 21.1 km as I wasn't going race pace but my body was used to running for that length of time so that, plus the other run workouts I'd done, meant I was ready to go as of race day. I was only a minute off from breaking two hours in that race, it was nerves and bad pacing prevented me from hitting my goal.
I know most of the rest of the FJ'ers here train based on mileage (is there a decent metric version of that? kilometrage just sounds wrong) so they'll all have differing advice.
Not that that that tangent had anything to do with your question. I guess I just needed to pontificate for bit!