Building bikes and taking names

Today I helped my friend Lena put her bike back together after she had it shipped here from home. One of the things I've had stressed to me recently is the importance of your stem angle and the spacers underneath it being in the most aggressive position you can be in. So when I saw Lena still had all her spacers in and her stem flipped up, I wondered why no one had ever considered lowering them for her.

Stem angle and the number of spacers underneath it determine the angle at which your body situates itself on the bike. For racing, you idealize the most aerodynamic position, which is low handlebars with no spacers. What confuses me is that I've heard this position is not "sustainable" for long distances. This of course makes me question what defines "long-distance." Is long-distance a cycling trip across the country or 112 miles in the middle of an Ironman? I would guess that because professional Ironman athletes don't ride with their stems flipped up, and 3-4 cm of spacing, it must be referring to the former.

While for beginners it probably is more comfortable to ride in this position, it also doesn't really train them to work their flexibility for dropping their body to attain that idealized flat back that cyclists talk about, does it?

Something I've been considering lately is getting professionally fit to my bike by our team coach. My concerns are cost and how terribly I really need it (but mostly cost). This 2-4 hour process will literally analyze every angle at which my body falls on the bike, among other things. I went to a local store and got "fitted" in about 20 min, which was how I found out about this whole dropping spacers thing to get in a more aggressive position. When I initially got my bike, I threw my stem in a pretty low position because I knew that I wanted my handlebars to be at least somewhat lower. Now, however, I'm faced with this idea of getting a new stem also because mine is adjustable which is... weird.

I wish this sport wasn't so expensive because I'm entirely too nerdy about it to handle the costs I'm faced with. In the meantime, let's ponder aerodynamics, shall we?