Nothing can take the place of hours in the saddle(seat)for conditioning yourself for a bicycle tour. OTOH, there is no need to over do it.
You can stay in reasonable touring condition riding a few hours a week, then as the leave date nears, gradually upping the distance. Do a couple of back to back 50-60 milers to get the feel. You'll gain conditioning as you tour.
BTW, touring is not a race. The average daily distance for loaded touring is 50-60 miles, but some manage much more.
Motivation, a bicycle that fits you well and a saddle that won't cause you too much discomfort are critical components of long distance touring.