I can't speak for everybody, but my first big 'goal' event was Goofy a few years ago. I know myself, and if I had done the traditional approach of running a 5K, seeing how it goes, then a 10K, etc... I never would have gotten the training in. I'm just too lazy for that. But setting a big huge stretch goal like Goofy, I knew that I had to train to survive (and with how much I paid, I was going to survive, dangit!), and I did
Granted, I ran two half marathons (including DL) and a marathon as part of my 'training' for Goofy, so it wasn't my first big race. But if you're properly trained for a Half, I really don't think that running a 10K the day before is much of a challenge since it's only a couple miles longer than a typical weekday maintenance run; I see no reason why this couldn't be the first event for somebody who put the training in for it