amy,

Just a word of advice not related to conditioning- If you plan to ride the MS 150, make sure you do several smaller organized group rides before and get comfortable with starting in a group and riding in groups because you will be doing just that. Since the ride is open to every one there will be a range of riders from absolutely never ridden before novices to experienced. The ride can be very chaotic as many of the riders are unpredictable so you need to be extra alert.

The other bit of advice I would offer is get used to spending TITS ( time in the saddle) time of at least 6-8 hours on a single ride since the first day is very long. The TITS time is almost more important than speed, at least in my experience.

You can totally do it and it will be a unique experience. Some people love it, others hate it and some change their minds from year to year.