I am confused as to what you want to achieve? Just the 100? Is your rail trial multi use? I have a hard time imaging wanting to do 100 miles on that, with kids and horses and strollers. Also why July? Why not later? Why not ride on the road? The best and only way to ramp up mileage is just to get on your bike and ride. If time limits that, then adjust your plans. I get the idea of having a goal, but you have to ask yourself why you want this goal? As it is not an organised ride, you can fix the day and date for whenever.