Same thing happened on my hike in Ocala this week. Ended up being a four-day hike instead of six, and that's only b/c my partner slacked me the final day. What is it about hiking partners getting blisters on the Florida Trail?
Getting back to the original topic of pre-hike planning:
Planning is fun. It's instructional. Planning and thinking about the hike gives you something to do in the long winter months waiting to go to Springer. It helps to give your family a nice spreadsheet showing exactly where you'll be each day for the entire six months. You don't have to tell them it's all a lie.
What helps even more is getting out and doing some hiking. A couple of long weekend hikes, maybe even a week on the trail, will go a long way toward making your hike successful. They'll also help with the planning -- you'll know how much food and fuel (and toilet paper) you need for the first section. Then show up in Georgia with 3-4 days of food, and one of the trail guides, and start walking north.