How did the stroke feel during the 400's? Did you hold the stroke together throughout the 400's?
One thing I have found is that when a swimmer is relaxed they can often swim better, when they are stressed or feel the need to push, then the stroke falls apart. Your stroke will get better and you will find it easier to swim further, it just takes time and practise.
Try this workout,
Warm Up
25m swim rest 30s
50m swim rest 30s
75m swim rest 30s
100m swim rest 30s
Main Set
6x 50m Drill use fins 1st & 2nd Thumb up side, 3rd & 4th Closed Fists, 5th and 6th Catch up, Rest 40 sec each 50m
3x ( 1x 25m Kick streamline underwater with fins rest 30s, 1x50m Swim hard with fins kick hard rest 30s)
1x 25m easy swim rest 1:00
6x 100m Swim with fins desending 1st aim 1:55 leave 3:00, 2nd aim 1:50 leave 3:00, 3rd aim 1:45 leave 3:00, 4th aim 1:40 leave 3:00, 5th aim 1:35 leave 3:00 and 6th aim 1:30 leave 3:00
1x 200 swim no fins Aim 3:06 rest 30s
Cool down
4x 50 swim easy any stroke Rest 40s
Total 1800m
Let me know how you do. The desending set is about pacing, let me know how you feel during each 100 and how you do with the times. The 200m at the end will be tough
Can you swim any other strokes well?
Swimming only freestyle can cause muscle imbalace, it is good to mix things up even if you just use another stroke during warm up and cool down.
Have fun
I will be off line until Tuesday, If you need another work out just revisit one I have given you and let me know which one and how it goes.
Have you had any problems with leg cramps during any of these sessions? How is your back?
Take care and have a good weekend,
David

