Look under the training section at weight pull, there is alot of info there.
First of all since you are just begining do not worry to much about going up in weight. I would stay with the drag weight right now and not worry about the cart yet. Thats just something else to redirect his focus. What I would do is maybe go up a little on the drag weight although a good rule of thumb is to drag 1/2 to3/4 of your dogs body weight.
That being said, short or long drags? Well it all depends on what you are wanting to work, fast twitch or slow twitch fibers in your dogs muscles. Actually combining the two on alternating days is good. It is just like weight lifting for humans, low weight long reps builds and tones muscle, short and heavy builds bulk. However it has been my experience and personal preferance to do lighter weight, longer drags. Now long drags to me is anything over 1000ft. My 55lb dogs used to pull 55lbs 2000ft but that was something we built up to over time. I like long and light because its my belief that it completely works the muscle and tears it down better, thus building it stronger and better cardio for the dog.
This will help you get started but pay attention to your dog. If he is running at the end of however far you pull or is pulling and not breathing hard, up it 5lbs. You want the dog to be tired at the end of the pull BUT NOT SPENT! Its up to you to find this balance.
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