I try to take a multi-week tour every summer (I'm a teacher.) It's always my intention to get in good shape first, but it usually doesn't happen. The end of the school year is my busiest time, and getting away for one ride per weekend is often impossible.
Sometimes I delay my trip until after the 4th of July (when I can never find a campsite anyway) and ride as much as possible between the end of school (usually mid-June) and then. That works pretty well. Once I left in my truck from home (central coast of California) to the starting point of my ride (north central Washington) right after school was out, but stopped to ride along the way - one day of driving followed by one day staying put in a campground and riding in the area. That was fun!
Sometimes I just go and start my tour in whatever kind of shape I'm in - not very good.
I always seem to have to start easy on tour for a few days, no matter what kind of shape I'm in. In years when I'm in pretty good shape I get "into the groove" within 3 or 4 days. From then on I average around 55 miles per day. When I start out in poor shape it may take a week or more for me to build up to this average.
For many years my itineraries have been very loose - no deadlines, lots of times no set end point; I ride until I feel like I've had enough then figure out how to get home. The one time I did have a date I had to be finished I felt some pressure to grind out miles every day, and a few regrets.
The biggest problem with not having an official destination is it makes me more likely to abort my tour early. You know how there are often moments on tour when you wonder, "Why am I doing this?" or think, "I wish I was home," or "I'm ready for this to be over?" I always get moments like that, but they always fade. However, when you have no official endpoint, it's easy to give in to those feelings in a moment of weakness.