Swimming is different from biking or running in that it's easier to go hard every day without tearing up your ligaments and joints. Since you don't feel the same type of soreness as when you run and bike, swimming leads many triathletes to think that maybe they didn't swim hard enough. Beware: Swimming hard every day eventually will wear you down and something will give. It could be your shoulder, your back...or your motivation.
So how do you manage your intensity when you feel good so much of the time?
Swim with a plan: Each time you get in the water the workout should have a specific purpose. And every swim set within that workout should have a distinct purpose.
Drill Sets: The purpose of a drill session is to practice technique and improve your efficiency. It's NOT to see how fast you can swim 100 yards while doing the finger tip drag drill.
Speed Sets: These sets are designed to teach you how to become efficient at going fast and to break the habit of defaulting to the same old pace day after day. These sets are NOT designed to see you swim 35 seconds for 50 yards and then swim 45 seconds for the rest. Pick a speed you can handle for the duration of the set. Typically, when I have a set of 10x50 yards, I start out at 40 seconds and work my way under 35. Not vice versa. Your fastest swim should be your last swim. Learn to swim fast when you are tired.
Endurance Sets: These sets are designed to create a nice big aerobic engine that will let you swim at race pace for as long as you need to. These sets include 200-yard and up repeats. You may have to swim a set of 3x500 yards. Just like when you swim the speed sets, swim your fastest repeat last. If you start out at 8:00 for your first 500, and then swim 8:30, and then 9:00 you just blew your whole workout. Remember swim your fastest repeat last.
The goal of any swim set, be it 50-yard sprints or 800-yard repeats, is to swim each repeat faster than the previous one. The more you start incorporating this into your practices, the faster you will become.
The main point is to learn the importance of pacing in the pool. Take it out easy during the first few sets and repeats when you swim. As you warm up, increase your pace. Your fastest sets should be the ones you do toward the end of your workout.
Practice the way you are going to race and it will become second nature. You want to be the athlete doing the passing at the end of the race, not the one being passed.
Start slow and finish strong.
Train to race: Sign up for a triathlon.