Apposite wrote:How hilly is the route?
With my very limited knowledge of cycling I'd say you are going at a decent pace, even though your route is short enough.
If you're reasonably fit and you are pretty bollocksed when you arrive that is a non-scientific way of determining that you are going at a decent speed.
Why not change the knobblies for semi-slick MTB tyres? You'll be amazed at the difference.
In that case, 21km/h sounds very good. Switching the noise converters for slicks is the easiest way of making a big gain.
To work out watts per kg you need a powermeter or something that simulates one like TrainerRoad.com. Then you do a painful test called a CP20. If you want to do one without buying any kit you should be able to find a coach or uni sports science lab with a Watt Bike or Computrainer near you that will let you do one for a few quid. Test itself takes about an hour with warm up and cool down. The key (and hurty) part is a 20 minute interval where you try to hold the highest constant power you can sustain for 20 mins. It is difficult to get the pacing right on the first few goes, you tend to save yourself a bit too much and spike at the end or bury yourself early on and blow. The result of that test will give you your critical power for 20 (CP20) and 60 mins (CP60, which is 95% of CP20). CP60 is also known as your FTP or functional threshold power. Then you divide by your weight in kg.