I almost killed the dog

Poor puppy! He is one big, fat mutt, and so I decided he needed to start working out with me. I took him to the pond to run with me, and he thought it was great for about 3 min. His pace started to slow after that, but it was not until about 10 min into the run/walk that he started dropping back. I was pulling him by about 15 min, an added bonus of a workout on my right side. At 19 min I looked back at him and his tongue was nearly touching the ground, he was limping, and he looked like he was about to pass out. I decided I could stop for a minute to take him to the pond for a quick drink. He decided to just jump all the way in. I had to let him, or I was going in the stinky pond with him. After about 3 min I decided he was stinky and muddy enough, so I pulled him out and made him finish the last 11 minutes with me. He was not loving it. At one point he almost killed me when he just laid down right in the middle of full stride. The landscape crew that was working on the pond got a kick out of me literally dragging this huge dog behind me. By 30 min he was done, and I was a little afraid he might have a heart attack. I let him lie in the pond a bit more. After I finally pulled him out, walked him home, bathed him, and let him back in the house, it took him about 2 hours to stop panting. He just laid on the floor the whole rest of the day. The next day was a walking day, and I worried about taking him, but I did it any way. He got a little pooped at the end, but handled it a lot better. I really felt bad for the little, big guy- But at least I am in better shape than someone!