bx, this is a thread I can relate to, thank you for posting. I'm about 5 years into self taught TI via videos and forum, started swimming only after retirement. I have swam 300M in the past but it was all effort. I have regrouped and refocused over the last year and am currently at 150M with little muscle or breathing stress. I feel as you have that I am just around the corner to swimming 400+ continuously. My journey has been one of turning focus points into repeatables, a long process, it hasn't come natural for sure.
I look forward to posting my successes.