It is about 12 km and took me about 4 hours. The slow speed of walking is because I kept taking photographs with my Canon PowerShot S3 IS all the time.
In June 2007 I did it again, but replacing the Kamppi-Koivusaari route with a pass through Taka-Töölö to add a few more kilometres, and to be able to photograph more scenery.
Sometime in sumer 2008 I'll do it once again, this time bypassing the islands entirely, and walking the whole distance on the continent. It will only add about two kilometres in Helsinki but over three times that much in Espoo.