I'd appreciate a bit of help if anyone can give it...

Last night I was attempting to play a campaign (with the English) to its bitter end by taking the Middle East (only the Turks remaining).
I'm having difficulty finding a combination of troops that are good in the desert when attacking. By the time my troops get to the other end of the map, they're knackered despite moving at a sedate pace (OK so it's a bit warm I guess). I've resisted the temptation to use the most heavily armoured knights etc, but even the archers were whacked out.
I've got to the top of the tech tree, but I'm not sure that's an advantage.
I was wondering whether using my older high valour troops would be better as they're less likely to rout when they finally get the oppotunity to attack.

I seem to be feeding my troops in to a grinder at the moment.