first spinning class sunday...any advice

Yup, try to follow the class (at least at first) but certainly, go at your own pace so you last the hour.

My only advice is you know they are like fixed gear bikes, you can't stop pedaling and coast. to slow down you have to slwo down your legs.   my knees will attest to that :^

From my own personal experience I would say that the instructors in most of these spin classes have the class stand up to much on the bike. They are really focused on the aerobic aspect more than good cycling. When I take the classes I change the routine a little bit to fit what I am looking to accomplish. So if I was working on hills I would increase the resistance on the bike and stay seated occassionally coming out of the saddle. Another thing that these classes do a lot are jumps. When they do jumps I usually just move my hands to aero position from where the bike breaks would be. Not saying this is right or wrong. I just think that this would be more beneficial than "jumping" on a bike.

congrats on going, the first class is always the toughest...

 make sure you get there early (10-15 min) and ask the instructor to help with the bike set up...maybe even alter it to fit your tri style (lower handlebars or whatever)

 bring a towel, (spd shoes?) and water bottle it will get hot

my 0.02

 I've done just 2 spin classes, and just in the last 2 weeks and I already agree with this.  My first class was with a tiny barbie-doll instructor who had us jumping up & down out of the saddle (not so good for boys in loose shorts) and it felt like an aerobics class on a bike.  Second class was with a 4x Ironman finisher who kept us in the saddle most of the time except for "climbs" and worked us by adjusting resistance/cadence.  While neither class was a whole heck of a lot like being on the road, I feel like the latter class contributed more to the kind of training I was looking for.

Wow, 1:30/100yd avg swimmer w/ 5 tris logged, that's great!  I'm sure you'll have no problem with spin class, and remember you are always in control of your own resistance. I've been going to spin class 2x a week for 3 years, and I'm hooked big time...it's really helped my road biking, *especially* climbing hills, plus anything endurance.

You've already gotten some good advice about bringing water, also bring a towel (ideally large enough to drape across the handlebars). If you have bike shoes w/ compatible clips  that's a plus too--check it out after first class.  Likely you'll be doing lots of intervals, maybe 1:30 or 2:00 repeating ones with short breaks, with probably some portion done out of the saddle.  If you're lucky like me, the instructor(s) are accomplished *real bike* riders, and "frame" things in real biking terms (over the music), and strive to make things interesting and fun!

Btw, I'm not above doing the "jumps" and "hover" stuff in spin class too, whenever that comes up--all that is great exercise for your core muscles, even if there is no direct translation to real biking.  And a little fast spinning (seated only) as the "downhill" portion after intervals is good too--gets your "fast twitch muscles" going.  And that actually *does* happen when biking down real hills, thankfully!

Hope you like it, and I hope I can make similar progress in swimming as you...I'm at 1:55/100yd avg at the moment, after a 13 mo. from starting at 2:30!