Hi folks! Long time lurker, first time poster. I'm coming from a swimming background and a total triathlon neophite, so caveat emptor.
I'm sure I don't need to convince anyone here that masters teams often bring a lot of swim-specific coaching baggage to the pool. Back during my long lost 100km weeks we did a lot of what I would call junk yardage. Distance swimmers were stuck doing half the practice with things like 5x1000, decend by 200. I would say that this was in the 13 range on Andy's RPE scale (or interval+5 if that means anything to anyone). This junk yardage was all in the name of sport-specific endurance. All of these swimmers had excellent technique, some better than others.
Two major differences for most triathletes:
1) Luckly for all of us, there are far more efficent sports for general cardiovascular conditioning than swimming. Like cycling, or better still, running. Lots of reasons for this, but thats a whole other post. But if you're doing lots in these other sports, chances are you don't need as much of the 'junk' that the rest of us swimmers put up with. Or at least you'd be doing it for different reasons.
2) Most triathletes don't have competetive swimming backgrounds, and might benefit from better swim technique. Likely this is the limiter in performance more than swim-specific fitness. Most masters teams don't focus enough on stroke technique coaching. This is really an issue of economy - given the number of people in the pool there just isn't enough time in most masters workouts for that kind of one-on-one coaching. Get some. Get one of your ex-swimmer team buddies from the torpedo lane to give you pointers before/after. Especially if they're cute. Or, all things being equal, pick the one with a masters in kinestheiology (we can all hope!).
So, given that, what intensity should your swim workout be? Like training in any of the diciplines, keep focused on what outcomes you want to achieve. Think about each workout and how it gets you there. Talk to your coach about which days are slated for VO2 max, which are threshold training, recovery, sprint, what not. Most teams are changing these up on a daily and weekly cycle. Make sure you show up on the days which will benefit you most. Ideally you're doing range of activites that span a range of RPE numbers.
One caveat. Technique is very very speed dependent. You recruit different muscles and use slightly different body position at different speeds. One of the reasons we train with things like zoomers (ie, fins) and paddles is to maintain higher paces with less effort (as well as put those muscles under higher/lower load). If you don't at least push your race pace in practice, you'll never build the techique and strength required for developing your most efficient stroke. So when you're picking your weekly workout targets, dont skip the anaerobic day at the pool just because you think you don't see how it applies to a 3.8km swim. Thats like never running hills/intervals or staying away from the big ring on your bike.
At the end of the day, even if none of this is particularly relevant for you, heres a rule of thumb. First, listen to the coach; they usually have an effort target in mind. Baring that, at least for the main set, swim as fast as you can while maintaining perfect technique (hah! I wish I could) for the whole set. The whole set - this does not mean RPE 17. Don't worry about the next set/drill; if we're doing things right that's likely programmed as mini-recovery. You'll get faster of course. At some point you'll forget how to swim slow (I kid you not; ask a college swimmer to a 45 sec SCY length with good form - they sink).
If folks are interested in learning more, I offer up the swimming bible:
Swimming Even Faster by Ernest Maglischo. Good sections on both training and stroke technique. We can likely discard the butterfly technique and flip turns, but otherwise largely applicable. Especially the sections on designing workouts and training effort.
Lastly, at the risk of breaking all orthodoxy (flame retardant gloves on!), I'm going to say that a heavy diet of long-slow-distance training for swimming does not really work for most triathete swimmers. Unlike cycling, you can't just change gears and keep the same cadence! Water has one resistance and you gotta have the speed to keep your form. Plus there's that thing about efficiency of swimming for cardio fitness. Our goal with swim training (most of us, anyway) is improving stroke efficiency primarily, sport-specific endurance secondly. Get most of yer LSD on the bike and run. In the interest of dialogue, I'd like to hear from other folks about results from LSD workouts for swimming, and where they're comming from in terms of swimming background.