Having observed the 1066 start and precicely how the Byzantines act against the seljuk invasion several times, I saw both outcomes- them getting utterly crushed as well as them celebrating triomphant victories against crumbling Seljuks.
The AI uses to rally it's massive army over the landway, marching trough all of Anatolia- In fact taking so much time usually (unless they are lucky and one of the Dukes is Organizer) that the Seljuks have warscore by then and are sieging the County of Melitene in the Duchy of Coloneia.
As The anatolian Mainland itself sucks hard attrition-wise, sometimes the Byzantine stack loses up to 2k men itself while approaching the enemy trough it's own territory.
Alp Arslan on the other hand is not lazy, at the start day he will raise his own mercenary force, the Ghulam (7k men, just as Mamlukes) and send them towards the Byzantine empire. They will start in Esfahan and usually reach the Duchy of Armenia Minor around the same time as the first seljuk stack (coming from the already assaulted holdings in the Duchy of Armenia), and the Imperial army (coming from foot trough Anatolia).
If the Byzantine Army manages to not starve in Anatolia and not waste too much time during the rally phase, they are usually in Coloneia before the Ghulam arrive, can attack 12kish Seljuks under Alp Arslan with an army of 12-16k men (depending on if the AI managed to get lack of funds already, in which case the Varangians disband as casual mercenaries do) and get a decisive victory (on the timeline around spring/summer 1067) over the Seljuks.
Given Alp Arslan has not died in battle, and the Seljuks do not implode (you yourself will barely have this problem as the for year old that is your heir is born in the purple AND has...at least definitely no worse stats then Konstantinos X does) you can count yourself enough together that after this victory the seljuks may have around 6000 men left, and 7000 Ghulam incoming, effectively outnumbering your army now. In case you are unable to chase down the first army faster, and the two stacks merge, simply run off to free the assaulted provinces, let them hunger down to 12000 again while they siege holdings in Armenia Minor now, and then strike at them with full force once the attrition made the numbers even.
Of course the player with aval usage has other options. You can easily ship in men to Edessa and Aleppo faster then the Seljuks march trough Armenia, and hold the score even with own sieges before the battle (do NOT assault holdings except cities, you will need every man for the battles).
as for the battle itself, I already mentioned the path of the First Seljuk army, which usually marches straight trough the duchy of Armenia, assaulting every holding on it's way. In Melitene, their assaults will halt as the attrition cost them so many men (they have around 12k here usually) that the AI thinks starving now pays off more then assaulting.
There is two options for the battle: the honest way and the gamey way with a bait.
The bait way is short and easy: You post 7-8k men in the County of Teluch as a bait, the other 7-8k you should field are on ships in the gulf of Antioch or what ever it's called.
The Seljuks, outnumbering your army, will attack with theirs once their siege at Melitene is finished, despite having to fight against a river rossing and mountain defense malus.
Obviously, you disembark your other half of the army once the battle engaged, and kick them back.
The honest way is to attack them at Melitene, hopefully with more men then they got, and hopefully before their mercenaries arrive. You must not attack from the County of Teluch- otherwise you suffer the river crossing mentioned above. Instead, try to circly them and attack from the County of Coloneia to only suffer mountain malus.
The simple reason to attack at Melitene is, they will, given you are even on strength, avoid your army and move to Teluch next, giving them the river bonus for every defense- and meanwhile they siege Teluch, their mercenaries arrive, making outnumber you by 3 to 2 again. So, attack them in Melitene, where they got no river and before the mercenaries arrive.
