I have never owned a dog before although I have done dogsitting/walking for friends' animals and get along well with dogs. I have 4 cats so would need a mellow dog who can get along with them. I am hoping to have a dog who will accompany me on daily walks as I get back in shape--have been doing 2-5 miles depending on my time and energy and hope to build up to longer walks. So I also want a dog who has the energy to walk those distances without being carried. Any suggestions?