If you are commuting, you don't need to cycle as hard as if you were training/racing. Just use a regular pace, like when you walk in the street instead of running. The same energy you use when walking normally will translate to long distances in a bike, because of the energy-efficiency of biking.
That's a good point. I'm young(ish) and tend to race my past self (while following traffic laws). On one hand, what's the point of biking if I can't get to work faster than by bus? But on the other, a more leisurely pace would surely leave me less sweaty while still providing some exercise benefit.
