"Trust" does not even enter the equation.
We have a mutual use for each other. Our use for them is logistical and intelligence, their use for us is making sure they're not a target.
The US will not give high level operational intel to the Pakistanis. They will give useful intel to us, because in the great equation of things, they'd rather sell out Taleban and al Qaedda than see the US turn against them. Pakistan has a lot of instability, a very large, very hostile neighbor to the south, and the possibility of economic isolation from the west and military attacks from the US is not in their interest. They realize this, so they'll cooperate. We need cooperation, not trust.
With respect to the "toughness" of fighting in Afghanistan, the primary issues are logistics, not combat. We do NOT need to hunt down every ******* who runs into the mountains with a Kalashnikov and whatever ammo he can carry. That's a USSR mistake. We will not abandon materiel and ammo on the battlefield like the Soviet troops did, they will not have the US supplying them with stingers, and the entire Islamic world supplying them with money, supplies, and personnel support.
If some ****er runs into the mountains and wants to spend the winter in a cave with whatever rations and ammo he can carry, let him. The bugs, worms and rats will finish with him come spring. The Afghan people will not be anti-US for long, the Iraqis weren't. Psyops, food supplies, and medical treatment, and respect for their culture and religion, will make the vast majority ambivalent, at worst.
Unlike the USSR, we don't need to set up an occupation or proxy government, nor do we need to secure control of the total territory. Once the Taleban leadership has fled, the Northern Alliance will provide civil government, they'll do the dirty work of hunting down Taleban remnants.
The Vietnam analogy is a crock of horsehockey. In Vietnam, you had another nation, North Vietnam, provide the major source of materiel, significant manpower, and you had the support of the two major regional powers in the USSR and PRC, plus access to trails and safe havens in two "neutral" countries the US was generally not allowed to operate in.
In Vietnam, they owned the night in most areas. The USSR had limited use of low quality NV gear in Afghanistan. We have the best quality night vision gear issued throughout our forces. When we fight, WE own the night, from the ground and in the air.
That gives us the ability to separate the fighters against us from significant sources of supply, that gives us the ability to hit them any time and any place they are on the move.
The Soviets made the mistake of attacking the villages in reprisal for supporting the resistance, they made the mistake of committing atrocities, they were indiscriminate in inflicting casualties, and they bought into the mentality of going after every peasant with a weapon. They still would have won without outside support to the Afghan resistance.
We have no reason to fight that way, we have a two decade plus advanatage in tecnology over the USSR army, and we have better troops, better doctrine, better leaders and better discipline than they have ever had.
In short, if Taleban wants to get it on with us, we're going to kick their asses so hard our boots will be sticking out their nostrils. The Iraqis were supposed to be ****-hot battle tested combat veterans. We taught them what real was was like. The Taleban will learn that same lesson if they give us reason to go after them.
They're threatening attacks on other nations who support us. That's already a confession of desperation. Whatever tough talk comes from them, they know they're ****ed if they take us on, that's why they have to threaten potential allies with what can only be terrorist actions, because they have no other power projection abilities.