Hi. It will sound like a strange question, but my muscles are getting really tired when I'm riding with my 5 year old son. Most rides are anywhere between 8 and 25 miles, with average speed around 6 mph.
On other hand, I can do 50-100 mile ride easily, without any discomfort with an average speed around 12-13mph.
The most interesting thing is that my Garmin 305 is showing more calories burned per mile on slow rides with my son that when I'm riding fast and by myself...
Can someone provide some easy explanation why its like that?