Learning to swim race pace 400 IM...Any tips?

Hi. Still kind of new here. And I did a search on "400 IM" but its said my terms were too generic and not usable in search.... Anyhow, figuring the 400 IM has surely been discussed, I thought I'd start a thread about learning out how to swim it when you only get to two races a year. I've swum the 400 IM as a target swim twice in last 6 weeks, i.e. warm up, ramp up 50s, recovery, then some splish-splash (roughly 1200 yds) then the swim. Then laying around hurting, then 200 ez. And then I'm tasting iron in my mouth and have hot, tingling toes and hands (this makes sense, I hope) for a good long while afterwards, so that's pretty much it in terms of my quality work for that work out. Getting to the pool 4 x week, it seems counter productive to do this much beyond once every 3-4 weeks or so. With so many variables (increasing fitness, weak/strong strokes in various quarters of the race, turnover vs glide, etc.) how do you approach things, not so much in the race, but in learning how to swim it your best? In other words, this seems like the hardest race to simply scale upwards and may demand the greatest amount of individualized strategy, but the success of any of the multitude of strategies only truly reveals itself at full on race pace. Thanks in advance, and, again, hope this hasn't been done to death....