Where is the ride and what is the terrain like? If the terrain is similiar to what you're used to riding it's possible. Try ridng 30 miles and see how you feel. Keep riding the next couple of weeks but don't kill yourself.

How many people are signed up? Realize, if it's a small group and the riders are faster you may be the last one on the road at the end of the day. OK if someone is watching out for you.

Do not spend a lot of time at the sag stops, no matter how bad you feel, it will cause your muscles to stiffen and make it harder to get on the bike. Do take food with you, it's amazing how sag stops seem to be lacking in good, sustainable snacks or you start bonking in between stops.

Also, don't think of the ride as 42 miles. Break the miles up, ie, 10 miles till sag stop #1, 15 miles until sag stop #2, etc.