Interval training, adding a variety of pull and kick sets and technique work are all valuable in helping you prepare for a long race distance, but getting in and churning out a straight swim, so that your mind and body know their capable of it, is also incredibly valuable. In your daily training, use goal setting to help you get to the point where you know you can handle the distance and duration. Here are a few different ways you might accomplish that.
Training the distance:
Find a long course (50m) pool or an open water venue so you can get away from the walls and get used to swimming straight longer.
Do a rehearsal time trial of your exact race distance and see where you are time-wise. It would be ideal to get splits of this time trial so you can track where your time might be falling off, whether you negative split the distance or maintain speed the entire way. If there is a pace clock nearby try to catch a glimpse of your splits throughout the distance. By splits, I mean maybe ¼'s of your overall distance.
Swim a 3000 for time and use the average as a threshold base for building sets to train from.
Create sets that build in distance and require you to get faster the further you swim:
1 x 500 on rest: 30, 1 x 1000 on rest: 30, 1 x 1500 descending your effort (so your pace on the 1500 is faster than the 500 and the 1000). As you get stronger and more confident continue to make the distances longer.
1000, 1000, 1500
1000, 1500, 1500
3 x 1500's
When you do break up a long distance into segments, try using short rest intervals, so that your body doesn't have time to fully recover. For your rest intervals you can use time (i.e. :5-:10 seconds) or sometimes I like to use # of breaths. For example, rest for 4-6 full deep breaths between repeats of 500's; this will encourage you to slow down your breathing and relax at the wall. When you know you are only getting a specific number of breathes, rather than a certain amount of time, you are more likely to avoid quick, shallow breathing. Staying away from a "time" interval also takes a bit of the stress, to make the interval, out of the equation. The athlete-turned-triathlete without a major swim background may really like this tactic or, use it when you are just getting back into training after a longer break.
Training the goal time:
Get yourself out in the open water for fun or for racing. The Splash and Dash series or the Colin's Hope Got2Swim Series are a great ways to safely get in an open water racing environment and maybe find some external motivation from the folks around you!
Swim open water for time – not necessarily distance. Head down to Barton Springs or the Pure Austin Quarry and just swim straight for whatever your goal time is. For example, you want to swim a 1:15 2.4 mile Ironman swim, just go swim for an hour and fifteen minutes at a steady moderate pace every few weeks or once a month. Swimming consistently at the same pace will NOT help you improve your speed so it is important that you do a variety of other training methods but, just knowing you can swim straight for 1:15 may mentally give you some confidence!
It is important to know what pace your goal time breaks down to and what it feels like to hold it. You need to train your body to manage the pace that you want to hold across the duration of your swim. Use short rest intervals while holding race pace; for example 75's, 100's, 150's, or even 200's on rest :10-15 seconds holding race pace. Build up the distances and the number of repeats as you improve. Another set that you could use to track your progress could be:
3 x 400 smooth/moderate effort on rest :20-:30
8 x 50's race pace on R1 1:00; R2 on :55; R3 on :50 (Extra :30-1:00 btwn rounds)
Use a tempo trainer to help you stay at race tempo. The Finis Tempo Trainer is one of my favorite training tools. Get an idea, through previous races or successful sets in training, what your best range is for your stroke rate and learn to train at it. This would be similar to holding a specific cadence range on the bike.
The sets above are the type of work you will primarily get at your masters workout and it is crucial for improving your swim speed. You should keep track of your progress in a log book and celebrate your improvements.
Of course, as a major proponent of focused technical swimming, I would be remiss to not remind you that efficient technique is equally as important. If you are not setting up a stroke that propels you forward you will likely be wasting a lot of valuable energy in the water.
Enjoy your time in the water and please do it safely.