I would like to add that the next couple of times you are out there with the same group, pay close attention to how things play out. These rides usually play out in very similar ways and improving your tactics will help you do better. We all have a limited amount of efforts we can make, so make sure you are using yours when they count and not wasting them on situations that'll get you nowhere.

For example (I'm just basing this on what you said, you'll have to figure out what works best) you say the group split in two on the first climb but then got back together. Maybe you don't want to try too hard on that first climb if the pack will reunite after it, conserving energy during that climb might be more beneficial. Same with those constant 'attacks' you mention. Try to not go too hard on those, make sure you're always holding on to someone's wheel and not fighting the wind. Then when you reach the decisive hill you will be fresher. Try to move up the pack and into a good position before the hill and find a wheel you think you can keep up with, and don't let it go. Again, this is just an example based on what you said, but when it comes to fast group rides (just as races) riding smart is every bit as important as your fitness.