Think about it conceptually
If a dog's metabolism is burning up food, burning up water, to where it has to replenish its fluids much more rapidly than normal ... the odds of it being a truly 'long haul' animal are slim
By contrast, if a dog's metabolism is such that it requires little food, uses almost no water, then the odd are that this dog will be able to retain its energy levels for a longer period of time.
It's the difference between one dog "burning its candle at both ends" while another burns very slowly ...
Jack