Can anyone offer any specific advice on how to move out to and beyond 100 meters?
From what you describe it looks like you just have a mental block, not limitations in your apnea. I've had quite a few in the past (coming up at 50 or 100 when trying to go for a big one), and never really found out why and how they unlocked.
Now I can't remember where I read this, but it may be something worth trying: if you come up at 85/90 because of anxiety, then you could try changing where you start in the pool, so you don't know where you are while swimming. For example start 13m away from the wall. Put a weight at the bottom to know when you reach 100m. This way you can focus on how it feels, instead of being just focused on reaching the wall.
I don't believe doing repeated 75m swim is going to be any helpful.
Other things you can try: make your training much harder than what you feel during the swim. For example very difficult tables, with repeated swim with only 10 or 20 seconds breathing towards the end of the table. or 10*25 meters with only one breath in between each swim.
Sprints are also a good idea, just to experience the lactic you only get at the end of the swim and be used to it.
Resting for a week before attempting a max can also help. Sometimes you bail out because you've overtrained, so you're not ready for a fight.
Contrary to what Dave is experiencing, I find it doesn't get physically easier at any point during the swim, for me it just gets progressively worse - until I give up because my legs hurt too much, because I've reached my PB and some, or because I bail out because I'm fucked up mentally. The only thing that gets easier sometimes is the mental part, when I get over the hump at 75/100 and fully commit to the swim.
Cheers,
Tanguy
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