I had a fitness test at Cycle U today.
Over 5 minutes and 15 minutes, your average output wattage is measured and provides the zone ranges (goals) used for training.
Ideally, it increases over time. I'm not exactly sure where it eventually (must) will plateau but my numbers went from 280 to about 315 for 5 minutes and from about 240 to 270 for 15 minutes. That's about a 13% increase after 5 weeks of training, so I'm pretty happy. It puts me in the range of 3.5-4 watts/kilo. Wikipedia has more on bicycle performance.
What is a watt? Putting "watts" it in caloric terms, my 5 minute "sprint" was: 315 watts * 300 seconds, or 22.6 kcal. Not a whole lot of energy I guess. Compare this to a car that produces 25,000 watts: Cars are very inefficient!
And I weighed myself and I've gained about 4-5 pounds the past 3 months. Which is great news, but I'd like to gain another 5 or so.