That tired, fried swimming feeling

I'm gearing up for a 400 IM race in a few weeks, hoping to do a little under 3 minutes slower than Ahelee (I wish!) (but note: I said "slower) (I'm aiming for 7:35 to 7:40 for long course). I've been training hard and consistently four days/week, 3500 meters or so each practice, but we've mostly been doing distance freestyle for the last few months. I've been diligent with practice for the last nine months (thank you, posters, who help me get to the pool even if I am blue) (:applaud:). On my own I've gone an extra day now and then and done stroke drills and 50s or 100s of stroke. So if I dive in and underwater dolphin at the start of the 400 IM, am I setting myself up for oxygen depletion and fatigue during the fly? I'm wanting to save my arms a little. I know that the best swimmers have no problem with doing as much underwater kicking as possible. But my kick is not superstrong (though I've been working on it). What I'm wondering is, even today, when I'm so tired in my muscles, is that from oxygen depletion? I have really good endurance but in races and often at practice my muscles just feel incredibly tired. I drink water during practice, but I have been struggling with eating well, and sleep is kind of useless many nights, so these obviously could be factors. But even when I've eaten and slept well, I am often very tired, rather than very strong, when I swim, though I always push myself and have gotten faster. I'd just love that really amazing feeling of power I've had maybe 3 times in the 9 years I've done masters swimming, where I feel I can push myself hard and I feel strong, rather than feel that I'm swimming hard but with a lot of muscle fatigue. So do you think it's a good strategy to save my arms but use my legs off the blocks and have less air, or is it a toss-up for me, perhaps?