Did my first group ride back in January and it was 48 miles. Riding in a group is hugely different, so make yourself known to who ever is in charge(i emailed the guy before i went) and they slowed it all down to gauge my fitness levels, and also told me where to ride. After a couple of weeks i was told to move closer and i will now ride within a few inches from the guys back wheel if it is safe. So look for someone to tell you where to ride, is not then i like to sit behind the front person on the left.
Decending i always look behind before i go into a courner and dont just sweep across, i try to keep my line pretty much the same if i can and try and follow the guys line in front of me. I also go for trying to keep up with the group, but often on long decentsi get left behind. You will soon get very confident at it and be going 40+mph down a hill wanting more speed.
Going up hills, the people should move round you if your slow, but if you find you want to go faster then look behind and check there is no-one there before you go to overtake. If someone is there wait and if they dont go then i point to go out and see if they move back slightly. If they do then go for it.
Dont worry about going up the hills, you might find that someone slows down with you to give you some help getting up them, and that the whole group might stop at the top of a big climb for everyone to relax some more.
And if its on a club ride then you will find you stop at a cafe just over half way. I try not to stop for a pee, but if i do then i do it when other people do, or if someone has a puncture. As for time limits most of the rides i have been on havent had them, a few have been speeded up because people have needed to get home for work or other things.
Not sure about the size of the chainrings, they are the normal ones on the Giant SCR4, thing i found was when people are on a triple and i'm using a double i find it harder to go at their pace up some hills.