either use the datefix tool to play past the game normal end date http://www.totalwar.org/MTWFiles/Tools.shtml

or if you want to begin the game early,
you edit the start date in the campaign
file's here
..\Games\Medieval\campmap\startpos

//========================================
//Start Date
//The year in which this
//campaign will begin
//========================================

SetStartDate:: 80
SetPeriod:: EARLY
SetEarlyPeriodStartDate:: 80
SetHighPeriodStartDate:: 600
SetLatePeriodStartDate:: 1000