In my experience, my upper body recovers a lot quicker than my lower body. Good news.
Yours might not, but I definitely argue that for 1/2 Ironman or Olympic/Sprint swims, going hard in practice is highly underrated.
There are few different kinds of hard, some of which might be termed fast in lieu of hard, but each type has a really important role in your workout.
1. Short rest intervals. With these, you can only go so fast. Going faster in this scenario would require more rest or else you wouldn't be able to do very many. It would be pointless for me to recommend a number of these for someone reading - I have no idea what you're capable of. But, Scott and I generally do Short Rest intervals as 100s on the 1:15 - maybe 10 of them. If we're really feeling good, it's 5 on 1:10. To make the latter, its a lot like splitting one of our better straight 500s. In general you're looking to get about 5-10 seconds rest. If you're feeling on point, 5 seconds is your best bet.
BEST TRAINING BENEFIT - the punchy sections of any distance, Sprint distance swims, and for holding it together during an olympic or 1/2 Ironman swim. You could call this threshold swimming.
2. Another kind of interval is relaxed interval training. I like to think of relaxed intervals as relaxed fit pants - they still resemble tight fit, but you have some wiggle room as to what you'd like to do. For Scott and I, it'd be any number of hundreds on the 1:20 or 1:30 depending on what kind of day we're having. Ideally, this interval is one where you should be able to go all day long, feeling just challenged enough to keep in interesting, without it being too slow that you could consistently do stroke or IM. With these it's fun to descend your time (not interval) or alternate fast/ez. I will argue that even though the interval is slower, you should still be somewhat aggressive. If on the 1:20 or 1:30, Scott and I will hover around 1:05 or 1:10 for our 100 times. Don't do less than 8x100 at this interval.
BEST TRAINING BENEFIT: Any - this is kind of the sweetspot. It will increase aerobic enzymes, "burn out the gunk", and generally make me feel more confident in my fitness. For those doing masters swimming, this is where most of the time is spent. Unfortunately, sometimes too much.
3. Fast Interval Training is next, and should be used for raising your top end, working on Lactate Tolerance, and getting yourself familiar with pushing your limits. Doing this every once in a while will keep your edge for when you need to surge in a race or training. For Scott and I, this is usually done as 100s on the 3 minutes. Yikes! Lots of rest. Yeah, because you're supposed to lay the hammer. Generally, Scott and I will aim for completing these in less than 1 minute. Done in numbers from 6-12, you're looking for best average here, and to feel the pain. Not intended to feel relaxed. About the last length of the 100, you should have doubts as to whether this is healthy.
BEST TRAINING BENEFIT: Beginning of a race, broadening your comfort zone and shifting your entire swimming speed capabilities upward. Sometimes you gotta throw gas on the fire. Lou Hollander, an 80 something year old ironman said the key to his longevity in the sport is doing something anaerobic everyday. That's what this is.
Again, one thing I like about doing more quality, along with technique in the pool is that there aren't long yards that let your technique break down. And if you're doing really hard efforts or sweetspot efforts and focusing on technique, then you're likely to become strong enough to hold technique (with focus) during the longer events. I would never recommend a set over the distance you're aiming to swim in a race - i.e. 20x100 for Olympic or 2500 for 1/2 Ironman. You have other things to worry about. Besides, if you regularly swim 3000 three or four times/wk, when you get to the 1/2 Ironman swim, the speed you'll hold for 1.2 miles will be slightly slower than you do your intervals. Swimming faster negates the need to prepare for something slower and longer IF swimming faster is making you stronger. Since it is, the effort and strength required to maintain a certain speed is less than it was before you were doing quality training.
Some people don't understand this. If I swim at a 1:30/100yd pace in my race, I should do different types of intervals (seen above) where I am consistently, with short or long rest, going 1:20s or lower. After a while 1:20 pace will become more regular, I'll be able to do more on the 1:20 over time and 1:30 will be easy to hold. And your body can take it, too! Cycling less so, and running even less. Swimming is not contact and its truly awesome. Enjoy.