Advancing my swim

I would increase N to 10-15 (so your main set is as long as your race distances), then look to shorten your send time (hopefully speeding up some in the process). 

It is going to be difficult to make substantial progress by only swimming 1x per week, though.  Ideally, you would want to be able to take a few months and swim at least 3x and work on a number of different things with your swim (including some coaching, if feasible).  Perhaps something to consider for the off-season before you look to do the HIM if it is not practical for you to do now.

1st: I can't stress enough how important it is to get  a coach to help you improve technique, in particular if you're BOP that's where you can find the biggest improvements.

That said, every workout should have a purpose. Discarding technique, you can work on endurance, strength, speed and overall fitness. You should work on these over a period, not just one workout. On the other hand, it's not fun to just repeat the same workout every week.

For endurance you should do longer intervals, for speed shorter intervals. You can alternate, say one week do 8x100, next week 4x200, then 10x100, then 5x200, etc.

Also, do some speed work for variation, say every 4 weeks, for example:

2x (25 - 50 - 75 - 100 - 75 - 50 - 25 10sec rest between each) 30sec rest

where you go max on the 25, hard on the 50, normal-hard on the 75 and easy-normal on the 100.

Or, you can do these as 4 sets or 4 repetitions of a set with one of each:

4x25 MAX/100%

4x50 Hard/95%

4x75 Hard/90%

4x100 Normal/80% <- no, not easy!

The time for each interval should leave you with 10-15sec rest, not more.

BR, Erik

Erik.  Just wanted to take a moment to thank you for your posts.  I always learn good stuff from you.  (Although this one makes me feel like a complete slacker.....I need to pick up the effort some)

Maybe I should clarify what's the point of above: 

You need to build up both total distance and the distance you can go without rest. So, first up the total distance with more intervals, then next maintain the same total distance doing fewer but longer intervals. When you get to 1200 total you may try 4x300 etc.

Regardless of the length of the interval, rest should be the same, about 10-15sec. As you get faster, shorten the interval time: If you're leaving @2:30 now, then when you can consistently hold more than 15 sec rest it's time to try 2:25.

As you do longer workouts it's easy to forget about speed, but you need that in particular if you only have limited time at each workout, and then speed workouts are fun If the main set of your last long workout is 1000, then you can do a speed workout with a main set of 800.

And when you do max/100% I mean, as fast as you possibly can, and then some more, forget about the interval coming next. 

I guess I will have to figure a way to get to pool another day because I am definitely not giving up!

Thanks to everyone for the advice.  That is why I love this site.

Just wanted to update in case other newbies were interested:

I still can only get to the pool once a week, but I have been able to increase my distance to 1500 meters non-stop - but slow...last time was 42:15.  I will say that when I am done, I am not tired or even winded, but always felt like I couldn't swim faster - I was locked into that pace.

So this week I took Erik's advice and did the first interval set he suggested

300m warm-up

25-50-75-100-75-50-25 x 2 sets

300 m cool-down

Did it all (1400m) in 39:15 the first time - and that includes the breaks.  Thanks for the advice Erik.  I look forward to improving even more.  Definitely winded and muscle fatigue now, so I know its working.

Signed up for an Olympic last night too.

I saw big improvements with swimming 4+ times a week.  Even if its only 30 mins it made a difference.  I actually saw more improvement with less overall time but more frequency.  Ideally, you need to be swimming at least 3hrs+ to see big improvements. But even if you get in the pool two times a week you will be better off.  I would focus on swimming hard has possible to make most of your time.  Quick WU then right into your Main set.