Loving the replies guys, honestly didn't expect I'd get so many.

@woodcraft
No, I don't get back pain when I'm alone. That's why I suspected the high speed/attacks to be the culprit rather than bike fit. I don't go that fast, nor do intervals.

@TimothyH
It really was a great day, I've rode with these guys before, but I was just getting started and usually get dropped. lol Actually finishing with the lead group in sight was quite awesome for me.

@rm -rf
Thank you for the advice, I'll try to work on pedaling faster rather than harder. As for drafting, we ride in two lines and each of us do his turn in the wind. But some of them stay behind. I'm not sure what the technical term is, but I'm sure you know what I'm talking about.

Spot on the tires as well, I have schwalbe lugano and found a rolling resistance test here about them: Schwalbe Lugano Rolling Resistance Review

They gave them a 1/5

@PepeM
Thank you! I'm sure once I'm riding with the guys more, I'll figure out their tactics and act accordingly. This was the first time riding with them this long.

@practical
Main reason is to get better, stronger and of course, faster. I loved every pedal stroke of the ride.

I'm fine with the fact that their better, I know they are. They're riding for years and I' didn't even finish my first. They're also the ones that are attacking and raising the pace, I'm just looking to keep up. I'm sure you wouldn't like getting dropped, would you? they asked me to join the local club, so who knows, it might get competitive and I end up racing, which I'd love to try.

@UnfilteredDregs
Ha! We already have a screamer on the group, I don't think he'd like another one. Thank you, I'll try to focus on my cadence more and see if that reliefs the back pain.