I haven't trained extra before a tour. Instead I've been using the bike to commute. In 2001, I sold my car and went cycle touring for 365 days. On return, I decided to wait a while before purchasing one again. That wait is now over 11 years and I still intend to purchase a car someday. However, it also means traveling many places by bicycle in between tours.

What I've also done is schedule some slightly easier days earlier in a long tour - with expectations that I'll use start of the tour to train further for the rest.