I'm in Week 3 and has a similar experience! I have two dogs, one an older Pitt/Sharpei mix and a smaller Papillion mix who is overweight. The Pitt mix I had to leave home during running days. I'll take her with for walks on "rest" days though. Her joints just can't handle it. The little dog I thought would be able to do it, but he was dragging yesterday on the second half. I was able to let him just trail behind because I was using a more flexible leash. I'm hoping he can continue to improve like I have and keep up, if not I guess it'll be walks for him on rest days too!