I've just done a similar distance ride and it was no problem and as long as you are reasonably fit then it will be a breeze.

Generally these things are broken down into 10-15mile chunks with the whole group regrouping and resting at the end of each. It means (as someone else said,) that if you are competant on a bike and reaonsably fit then you get a lot of rest during the day.

Good Luck,

K