Yeah, I wouldn't try doing the whole lot with a sander. I've only had to add a forearm contour twice and a gut contour once, so I'm hardly a definitive authority on the subject, but in all three cases I got most of the bulk out with a simple wood saw, then a big rounded rasp to blend in the edges, and finally an orbital sander with medium grit to do the fine shaping; I always do fine grit sanding by hand.
A forearm contour usually drops to around half the thickness of the body blank – that's a lot of wood to get through with a sander, even with very rough grit. Plus it's a lot cleaner if you can get most of the wood out in one lump rather than having dust and splinters flying everywhere. Draw your contours onto the body, saw or rasp off (I can't vouch for a chisel; it could work but I usually use softer woods that get damaged too easily by chisels and avoid them) the majority of it just a couple of millimetres above your contour lines, then break out the sander to get right down to the lines.