I love danes!

As w/ any dog, you don't want the cart too heavy nor do you really want them to pull before the age of 2 to make sure their hips/overall health can accomidate that.
If I remeber correctly Glacier already explained that.

Due to the fact that you should wait until 2, I think a dane would not be the perfect choice, their life span is fairly short, with the health problems that *most* dans encounter, they probably would be very limited the last couple/few years. That only gives you a couple years of pulling.

Also w/ extremely large dogs you have to consider that they are prone to bloat. You would have to juggle their eating & jogging times just right.