For me, speed is important because it is a function of my heart rate. I ride because I love cycling. I have already put in close to 1500 miles since January 1, 2010. However the cardio fitness of each ride is important to me and I want my average heart rate to be in cardio zones 3 and 4 and my maximum heart rate in zone 4 or 5. In order to reach my desired heart rate zones, I need to push myself and give myself as much speed as is possible.

To answer the question, the desired speed you want is the speed that will place you in the heart rate zone you wish to be in. The speed will be different for individuals because of the differences in height, weight, age, fitness level, quality of bike, terrain and weather conditions.

And for me, a bike computer with a heart rate monitor is vitally important so as to provide me with the data that allows me to improve my cycling skills and fitness level.