Hi razerbladefya,
You can change the end date by entering your game's campaign folder - usually either "C:\Program Files\The Creative Assembly\Medieval - Total War - Gold Edition\campmap\startpos" or "C:\Program Files\Total War\Medieval - Total War\campmap\startpos".

Once you have accessed the folder, select the textfile of which ever era you wish to change. As you may expect, the file name corresponds to the title of the period, so there should be no problems locating what you want.

Once you have done this, scroll down to the bottom of the file, and add the line:
"SetGameEndDate:: y"
Where y equals the year you want to extend the game to.

Once you have added the line, save the file, and all new games should last up to the specified time. Saved games are not effected, so, unfortunately, the game will end as normal.

Hope this helps, good luck, cheers!

BTW, the Viking campaign already has the line filled in. In that case, simply change the number to what ever you desire.