Catriona, I know exactly what you mean when you say your Surly feels like a tank. I feel the same way about my Jamis errand bike. But, I love her anyway! I will never go fast on this bike, but I am always smiling when I ride her.
When I am on my Kuota, I feel the need to prove that I can go fast enough to justify me owning such a light bike. Not that it works...