I've been doing triathlons for 3 years now, predominantly short course with the odd 70.3. My annual swim program is wait until 6 weeks out from first race then swim 3 days a week for the duration of the season (4 months), then stop over winter.
Each swim session is usually 45-60 minutes, consisting of usually 1000m time trial (warmup), 800-1200 meters of 100m efforts, then cool down with 500-800m using fins. I sometimes throw in some laps with feet banded together.
I usually see myself go from 2:00/100m to 1:40/100m (50m pool times) during the course of the season.
Is there anything different I could be doing regarding the structure of this workout that would see my times improve further.
I don't want to do any elaborate long winded drills with swim toys, just freestyle. This is the most time I can commit to swimming.
Thanks in advance

1:40 pace is my 1000-1500m pace, I go just under 1:40 for my 100m intervals, on about 20sec rest. There's very little difference between my interval pace and 1000m pace

Thanks for the input guys. Will definitely take on some of these tips. My lack of swimming during off season is due to a shift in priority to the road racing season (cycling), I may have to be more flexible and swim once or twice a week for maintenance.

Well, the small delta between your intervals and your steady state is pretty typical of people who've never swum on a team and never really worked on their speed. Given the very limited amount of swimming you've done, holding 1:40/100 m long course for 1000-1500 m is actually pretty good. If you were to keep swimming thru your off-season, i suspect that, if you swam some long rest 50s and 100s where you tried to go as fast as possible, you could prob get your best time for 100 LCM down to 1:20 or so in a few months. As has been discussed at length in the "How fast do you need to be" swim thread, your distance potential is limited by your sprint speed. Once you get more sprint speed, your average pace for the short rest (15-20 sec rest) intervals will come down and so will your steady state pace.

I have a bit of the opposite going on, except for not swimming often. Took up swimming last year in September because I wanted to compete in my first sprint and Olympic distance races this year. Started at 2:20/100m (SCM) and being exhausted after 4-6 laps in the pool. Worked my way up to maintaining a 1:42/100m (SCM) average for 10-15 sets on 20" of rest by only swimming once, sometimes twice a week on my own (1-2k). Averaged 1:43/100m in last Olympic (first open water swim, with wetsuit though). No coaching. Decided I wanted to get more serious, so now swimming three times a week and I get swim coaching once a week.
Last Saturday the coach had us swimming 100m medium pace but paying attention to technique, and 100m all out where they would film the last 25m because that's when your technique will likely break down quite a bit.. First 50m I came in at 34-35s w/o flip turns, total 100m time faded to about 1:20.. So unlike OP, I would say the difference between my sprint speed and more or less steady state speed is quite large.
Would you say I would also benefit from doing 50's and 100's on longer rest (what is longer rest? 1-2 minutes?), or should I actually do longer high intensity sets (250m for instance?) or what would you suggest? And if you could explain why please.
Sorry OP, don't mean to hijack your thread :)