Have to agree with Tosamama. The breeds you mentioned are not really suited for hiking, and if the weather is too hot, the dog may stop lay down and not want to go on until rested. Also, keep in mind that you may encounter a predator (hope you'll never do) but it may happen after you've already hiked 4 miles or so in the heat, will your big dog have what it takes on energy needed to protect you if tired?