I've started walking again on a regular basis and my wife has started to join me (I'm so proud of her!). As I'm doing this I'm logging my activity into both MyFitnessPal.com and MyDailyMile.com so that I have a record of what I've done and how well I'm doing. It helps me to track calories burned and fitness levels.
One interesting thing I've found is that if I walk 45 minutes at a 3.0 mph pace I will burn around 500 calories depending on which tool I use to enter that information. This is a pretty good work out for me and I'm definitely tired when I'm done. If I bike for the same 45 minutes at a 12.0 mph pace the calculators show me as burning around 800 calories. I find this very interesting because I'm much more tired after the walking than I am after the biking.
I do realize that no calculator is going to give me exactly what I burn calorie-wise while exercising, but it seems to me there is a disconnect between the 2 activities. I have a feeling the calories burned for the walking are on the low side and the calories burned for the riding are on the high side.
Regardless of the actual numbers, the important thing is I am getting up and getting active and burning calories. I know it is doing god for my body and my health and I am committing myself to keeping up with both activities.