We have been lucky in this country thus far that most of these attackers have acted alone and, on balance, at best at the skilled amateur level, without an overarching plan to escape and continue a campaign, or with the intent to martyr themselves or suicide by cop for notoriety. 

Lucky because these attackers have shown how much damage and terror even they can cause in the short time until their decisions get them caught. The Beltway Snipers, who did a decent job of disrupting an entire city, spring to mind as an example of the kind of longer-term threat we could face. Just imagine them with real training, a coherent plan, and more resources.