First off I am training to run in a marathon. I am putting in about 30 miles on average a week. One of my Jack Russell's loves running short runs with me (4 miles or so). Is there any long term health problems as far as bones and joints I should be worried about? I would like to take him on my long runs (10+ miles) but I don't want to hurt him in the long run. I am pretty sure he would love it though.