Simply goto the campaigns page and hover the mouse over the button "
LONG CAMPAIGN" and read the tool-tip. That should give a little clue, no?
Now click that button, and on the long campaign setup screen, chose your nation and start date (if you don't want the default). And number of battles etc.
Also - in the manual, click on the linky entitled
Playing a Campaign and the destructions are tjere for all types of campaign - you want to pay attention to the bits talking about the Long campaign.