They have to get through Indian Army lines, too, and India outnumbers the Paks.
So, which army would it be easier for an infiltrator to avoid?
What's lost in this discussion is that al-Qaida would love to start a war here in order to provoke an attack against Musharraf. Musharraf has sided with the Great Satan, after all.
If they can get the Pakistani army destroyed and Musharraf out of power, they will have accomplised a huge strategic goal.