How do I widen a hole in wood for flex tube?
Now you've learned the hard way, the same way the rest of us learned, why they say "measure twice."

A spade bit may work for this trick, but an auger bit may be easier to control. First, use the bit to cut a hole in center of some scrap wood. A piece of plywood that's 4"x6" would work well, and a 2x4 that's 6-8" long would also work. You need a few inches on each side of the hole, and enough so that it doesn't shatter apart when the drill bit is vibrating inside, but thickness isn't that important. Since you're working in a crawl space, start a few screws on each side of the hole that you drilled.

In the crawl space, line up the hole in the scrap wood with your existing hole and screw it into place so it doesn't move. This wood is now your guide to keep the drill bit from running all over the place. Put your drill bit inside this guide, make sure you're aligned, and drill your larger hole.