For this second leg, concentrate on keeping your pace per 100 the same as you did on the 500, but try swimming slightly faster on the second 400 repeat. You may not have as much rest as you did after the 500, but keep in mind that you are dropping the distance you are swimming so your interval, though consistent in base, will seem shorter. That's part of the challenge.
The 300's are the middle leg of the set, and possibly the most difficult. Not quite endurance repeats (like the 500 and 400), but not short enough to be akin to sprint repeats (200,100), this set is the turning point for how successful you can be overall in this workout. The pace should be equal if not a bit faster than the first two legs you just did, though your effort should be at about 90 percent.
It is the longest leg of the set at 900m, and if you can make the intervals you are home free, but it might be touch-and-go between each 300, so be prepared to swim through with barely a break.
The 4x200 will be your first foray into short repeats. Though the interval base is the same, it will be tougher to finish 4x200 in a row after what came before. Swim to make the interval but be cautious not to shorten your stroke and get sloppy.
Swimmers often tend to change their technique when they shift into overdrive, especially if they feel fatigued. Keep your strokes long and powerful, but increase the effort to 95 percent if you can.
The last 5x100 is your chance to be explosive and fast at the end of two intense miles of pace training. While you may have felt like you were sprinting at 95 percent effort on the 200's, your times should have been at long-distance race pace at that point in the set, assuming your endurance needs work (that is what this set is designed to develop).
For the 100's, you may have to force yourself to sprint at 100 percent effort just to make the interval or keep your previous pace. But the repeats are short; you should have a few seconds rest after each one, and there are only 500 meters in the set as opposed to the previous sets of 800m or 900m.
Getting through this workout and successfully making the intervals is a challenge enough in itself. Once you are able to master it, you should see a significant improvement in the back half of your races.
Triathletes may also notice less fatigue at the end of their swim accompanied by a more powerful exit and transition into the bike leg.
An even more challenging approach to the above workout is to decrease the base-100 interval as you descend down the ladder. For instance:
1 x 500 @ 6:15 (1:15 base)
2 x 400 @ 4:50 (1:12.5 base)
3 x 300 @ 3:35 (1:12 base)
4 x 200 @ 2:20 (1:10 base)
5 x 100 @ 1:08 (1:08 base)
Obviously, this hardcore approach puts more emphasis on descending your interval throughout the set under the assumption that your endurance is already at peak levels.
The challenge here is not simply making the set and maintaining the same pace times throughout, but challenging yourself to swim at threshold and actually dropping your time (not just increasing your exertion effort to maintain it).
For novices that yearn for a taste of elite-level challenges, a more realistic approach to this workout may look like this in a 25-yard pool:
1 x 250 @ 25 secs rest (250 yards)
2 x 200 @ 20 secs rest (400 yards)
3 x 150 @ 15 secs rest (450 yards)
4 x 100 @ 10 secs rest (400 yards)
5 x 50 @ 5 secs rest (250 yards)
TOTAL: 1,750 yards
In general, ladder sets are favored by endurance swimmers because they avoid the monotony of typical endurance training where the same distance may be repeated several times (3 x 1500m, for instance).
Janet Evans' example is effective in that it's not only a ladder in the typical sense (500, 400, 300, 200, 100), but it adds an extra layer of complexity and challenge by increasing the amount of repeats as you are descending down the ladder.
It clearly worked wonders for her career; perhaps it can do wonders for you.