Sit back and prepare for the usual onslaught of quips, ridicule and witty banter!
Leave the computer on but put a peice of black electrician's tape over the numbers so you can't see them. That may allow you to gauge your progress without feeling like you have to take any unnecessary risks. Personally, going down hills fast scares the bejeezus out of me. For fitness purposes I'd rather hammer up them and brake going down, even though it will skew one's average speed downward quite a bit.
If you really want to gauge your fitness levels it's time to take the plunge and get a heart rate monitor. Being a fellow computer geek, you'll love it. Lot's of numbers and details to dabble about.