I'm new to cycling--I just started this summer--and I'm currently riding a low-end Roadmaster bike, handed down from my dad. I'm perfectly willing to spend money (once I get some) on a fancier bike, but I've resolved not to spend money unless I understand why I'm spending it.

So here's my question: Is there any real advantage in spending $500-$600 on a new Trek or $800-$900 on a new Breezer to replace my sub-$100 Roadmaster? Are more expensive bikes worth the money? If so, why? I realize that I am exposing my ignorance here, but I would like some more insight before I spend money.

Note: So far, the Roadmaster has worked well (for just under 1000 miles). My short-term goal is to ride in the MS 150 next September; my long-term goal is to commute to work (once my kids no longer need me to drive them to school.) I'm not interested in racing; I'm more interested in getting and staying in shape.

Any comments would help. Thanks!