If you already have a second set of circular needles of the same #10 size, then you can use the "small diameter knitting on two circular needles" technique (video in advanced techniques section). Just start knitting with the second circular needle, until you're half way across, so you'll have half the stitches on each cable needle. Then, continue knitting with the first cable needle, by using the other end of that same cable needle. Alternate between cable needles, always using the other end of the same needle. It's not hard to do.
I love my DPN's, but my needle collection isn't complete, and just a couple of days ago I used the two-circular-needles technique to finish a hat. I enjoy doing it this way too. It's good to know both methods!
If the lady at your local LYS was saying to use one long circular needle, then she was actually talking about Magic Loop knitting (video in the same section). Again, it doesn't hurt to know that technique too, and it will get you out of a fix if you've got a large circular needle and nothing else. I don't know if a 24# would be big enough to decrease a hat; it would be pushing it. Longer needles are better with ML knitting.