You call that a “really high-poly model”? chuckles
I eat models of that res for breakfast!
…ok, enough nonsense.
First things first, in the future you’ll want to avoid applying SubSurf modifiers your mesh until the final stages (if at all). Keep them on. But don’t apply until/if you need to.
For selection, if you do a [Select] > [Edge Loop] (while at least 2 verts are selected), that will select an entire loop if the geometry forms such a loop. This should work for things like limbs.
In this case it still may take a while, but pressing [b] twice will enable the circular selection tool (I forgot what it’s actually called). Then you can “paint” a selection of just the verts you want (you’ll need to zoom way in). [MWheel] to resize the circle, [MMB] to deselect.