I am of the opinion that it is better to be in shape starting the tour, rather than attempting to get into shape while on tour.

Attempting to get into shape while on tour takes the fun out of the first couple weeks of the tour. DOMS can be a real pain on about Day 3 of new exercise, and that's the last thing I want to deal with when I'm trying to relax and sightsee ... and especially when I know I have to get back on the bicycle and ride again the next day.

And ... getting in shape, and being in shape, is a good idea anyway.

I recommend cycling outside whenever you can ...
Cycling inside on the trainer (you can get a trainer relatively inexpensively) ...
Taking a spinning class ...
Joining a gym ...
Doing some winter sports like cross-country skiing and snowshoeing ...
Lots of long walks ...