There's no "set" training schedule for something like this. Some people, never having ridden 2 miles in one day, buy a bike from a big box store and just head out. Eventually, they reach the other coast.

Others say you should be able to ride 50 miles a day with 4 fully loaded panniers (roughly 50-70 pounds of stuff) for 3 consecutive days before you set out.

The fact is that, if you can ride for an hour at a time, you can go coast to coast with little to no training if you are determined and unconcerned about speed. After the first week (5-6 days of riding 4-8 hours a day over varying terrain), you'll have begun to get into "true" bike touring physical shape and should be able to complete the tour.

After Day 3 and/or about 100 miles, the only limit will be in your mind, your wallet or your actual physical health (hopefully not due to an accident, but illnesses have ended trips).

Best advice I have heard was to plan on taking the first 3 days easier/making them shorter than you planned average mileage for the entire trip. This somewhat limits your aches and pains while also providing you with some real distance from the start point.

Also, consider the difference in route distances such as the 3 ACA routes: Northern Tier-4321 mi (more or less depending on how you go around the Great Lakes); TransAm (across the central US)- 4250 mi; Southern Tier-3132 mi. Like said above, at 70 miles a day, you can plan on anywhere from 44-90 or more days on the road not counting rest days off the bike. Most people take 1 day off every 7-10 days, some more, some less.