AdrianTX,
Welcome! And good luck on your MS150 ride. Like strings&gears, I've ridden in the MS150 in eastern NC for the past 3 years. I'll echo some of the advice you've already been given:
1. Get in a lot of saddle time. In addition to building your endurance, it helps get your hindparts ready to be on the bike for an extended period.
2. Practice riding in a group if you can, and keep a watchful eye on the other riders. There will be a lot of experienced riders, as well as a lot of inexperienced ones. In my first MS150, I got caught up in a pileup caused by an inexperienced rider jamming on his brakes in the middle of a pack. I did a classic endo onto the pavement, and I can tell you this: asphalt doesn't have a lot of "give" to it.
3. Practice riding in a paceline, which can help conserve your energy. It's also a nice way to meet people and share the experience (and the workload) with other riders. There's a lot of great information on riding in a paceline at
http://www.tarwheels.org/pacelinev92.pdf
4. Since the MS 150 is a two-day event, bring spares of everything if you can. For example, I take two helmets, two pairs of shoes, two pairs of gloves. On my first MS150, it rained for much of the first day. My shoes and gloves were still soaked the following morning, so it was nice to have dry things to wear the second day.
5. Take advantage of the rest stops, but don't stay too long as your muscles will start to tighten up.
6. The road is pretty crowded between the start and the first rest stop and starts to thin out after that. I usually skip the first rest stop to avoid the crowds.
7. Most importantly -- HAVE FUN!!!