Bashed knee - healing time??

Hi all

Did a really stupid thing last week: fell down the cellar stairs and bashed the centre of my left kneecap on the post holding up the bannister.  Bruising started to appear around the edges of the kneecap (not where it was hit - that was a very small, localised area) so presumably it was bleeding inside a bit. 

I've been for a couple of runs (8 miles on Sunday and 3 miles this morning) and hurt a lot afterwards - swelling came straight back up and bending it didn't go past about 90 degrees, so presumably it bled some more.  I've been taking ibuprofen and solpadeine but no aspirin.

Does anyone have any idea how long this sort of thing will take to heal?  I'm climbing the walls at the thought of not being able to run, but really don't want to miss out on the marathon in May because of being stupid and running on a bad knee.