Whack that sapling down and make a bow. Don't worry about the heartwood/ sapwood thing. Both will make a bow, even a combination of both on the bow would be nice. I've only made one with Mulberry and it had little to no heartwood but made a fine bow.
I don't think you'll waste your time. There are so many woods to experience with different ways to use each one, so much to learn. Long as we are learning, its never a waste of time. It would be great to see your results, let us know how it turns out.