Train them like you race them. I've heard about training with heavy loads, or training by running a 14 mile trail 6 times a day, or some other permutation of what our distance dogs do, but my philosophy is to run them with a reasonable sled load as far as you can and still maintain them. I tend to ride the drag too much training and racing, because I have a conservative streak. I personally know how hard it is to patch up orthopedic injuries, so I try to avoid them. We have a tried and true program that starts August 1st and works up gradually to December.
Temperature is the early limiting factor.
I feel that 4 days a week is ideal. Our goal is to peak physically and mentally at the start of our main event. We want to be able to spurt up to 16 miles an hour 10 hours into a run for a short time. I can't list them all. Winning is contingent upon accomplishing all the tasks.