I'm sure these are large, strong, and healthy dogs. But what you are asking of them in terms of physical stress and calorie needs I think will be very hard on the dogs. I'm trying to be as tactful as I can because I think you are being sincere, but this could be abusive to the dogs. These dogs will need to be fit for the task. Would you ride a century if you didn't train for it?
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke