More swimming at your goal pace, most people try to hit pace on too short of a rest. Start with something like
3 rounds of

16-20x25 @ 25 try to hit 16 on all of them...
200 easy in between

I'm 41 and have recently swam
4:14 / 4:24 (scm/lcm) 400m - 2017
8:51.1 (scm) 800m - 2018
16:58.9/17:32.0 1500m (scm/lcm) 2017
4:51 400 IM (scm)
58.9 100 Fly (scm)
2:13 200 Fly (scm)
2:14 200 IM (scm)
2:15 200 Back (scm)
2:37 200 Breast (scm)

I swim 3-5x most weeks with most sessions in the 2500-3000m range, nothing over 4000m in the last while.

My gut says you need more top end. What's an all out 100 looking like?

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2020 National Masters Champion - M40-44 - 400m IM
Canadian Record Holder 35-39M & 40-44M - 200 m Butterfly (LCM)