Great 5km

I've been looking at the Run Less Run Faster book in more detail and looking to the future I don't have a recent time for a 5km run so, probably inadvisably I decided to run a 5km at race pace last night.  I know I should be tapering and will do now but I was curious.

After 1km at a nice slow easy pace for a warm up I let go.  The next km was 4:36 which was a good pace for me.  I kept going and hit the second km of the 5 in 4:26, slightly faster, and third came in at 4:28.  In my head I had the notion that for a goal for next year I would try to break 23 minutes for a 5km race, at my current pace I was on to break it now.  I pushed a little harder on the 4th and finished it in 4.18.  I was starting to get carried away.  A sub 23 minute 5km was definitely possible but if I could keep up the pace a sub 22 minute 5km was also realistic.  I pushed harder again and dug in knowing that the course I picked the final 100m would be up a hill.  I finished in 21:48 my faster 5km by almost 2 minutes.  What a great run.  Now I really need to rethink my goals for 2012, would a sub-20 minute 5km be too much to aim for?