I think it can be whatever you want it to be. When I was a runner I would often throw in a track workout so I could do intervals. It was a good compliment to my overall training schedule. It sure beat a treadmill, or in this case a trainer.
I'm not ready for intervals yet, and I KNOW if I go, I'll be chasing somebody. I have to force me to ride slow for a true base this year. Gloves come off in March--then I'm going _less_ slow.
And Mike and I can take up klling these hills again.