I ride my bike ~4-8 miles ~4-5 times a week to work. It's flat, road surface riding. I don't have a set plan for the rides; if I feel like doing sprints, I do sprints, and if I feel like taking it easy, I go easy. Today was sort of middle intensity. I wanted enough to raise my heart rate and flex my legs but not cut into my strength for tomorrow's workout.
I don't count these rides as cardio, more like background metabolic activity. Just another small thing to get the blood flowing. Here's a pic of my bike:
I had a big chain ring put on. I forget how many teeth it is, but I remember that it's the same size as most outer rings on road bikes, which is huge for a single speed. Most single speeds/fixed gear bikes have mid-sized chain rings to create a balance between speed and ease of pedaling. I wanted a big one so that I could ride fast but also maybe make it more like a strength exercise than cardio.