Man of Honour
seems a lot to me.
Garmin probably use a better algorithm for calculating calories burned as it takes the heart rate and stuff into the forumla they use if you have a HRM attached and I'd say these are still wildly exaggerated even though the device is supposed to learn the rider and get more accurate over time.
Your walking 12 miles probably at low heart rate can't be anywhere near cycling for 2-3 hours in calories burned?
Walking burns more kcal per mile than cycling as you're using more of your body and in a less efficient way. Obviously you can do a lot more miles in an hour cycling than you can walking so you can probably burn more kcal in a given time. Running is the fastest way to burn kcal though.