Are long runs really worth it?

Long run day today. Wanted to increase the distance from last week but not go to far too early in what some might call the building stage. I ended up running just over 10 kms, about a km more than last week but much slower, my legs were a little sore and stiff and I started out with two 7min kms up hill. So are long runs really worth it? Well if you are going by the smile I always have on a warm down lap of the block after a successful run then yes...because they make you feel GREAT!