A message to Karl, you have made a great choice for your first marathon.
Last year was my first, and so far only, but the organisation was top class and from all accounts this is about the most picturesque city run you could have chosen.
You have probably already read about the dreaded Västerbron bridge, which you have to cross twice - the 2nd time at ~30 km and the approach I took and worked for me was to walk a little before and over it for that 2nd crossing.
I followed a 16 week program, but for various reasons only managed 10 of those weeks and completed in under 5 hours and was ok to do a 10 km run on the Tuesday - so follow the program and you will be fine.
Just take it easy, take plenty of fluids and enjoy the day - finishing in the old olympic stadium is something very special.