As for distance per day, I'd recommend what MrToe has brought up in the past- that if possible, plan for shorter days for a day or two, as riding with a fair weight is tough at first.
Even without knowing how much you ride and what shape you are in, the "shorter first few days" recommendation is a good one and nice if there are campgrounds within 30, 40, 50 miles at first. It's just nice for the legs to get used to being a mule train.