Kind of echoing some others here but you really do need to be fitting well. You mentioned raising your seat 1" but are you sure that's enough? Maybe it's too much, maybe not enough. The point is, you'll be doing yourself a favor to get yourself fitted the best you can to your ride. Everything builds off of a good fit.
Next, be a spinner and not a masher. You're better off to be spinning at higher RPM with less effort than pushing harder at low RPM. Riding with better technique will enable you to do more miles, more comfortably and with less stress. Your legs - especially your knees - will be happier.
Finally, group riding is not the best for beginners. Yes, it's far more social and inspiring, but it's also the best way to find yourself riding "over your head." It's too easy to get pushed into higher speeds and longer distances than what you're really ready for.
Get yourself fitted to that Nishiki and get in some good solo riding time where you can work on yourself. In short order you'll be strong, comfortable and confident and ready for group rides.
-Kedosto