Even splits is NOT the best way to run the 400m from a physiological standpoint. That doesn't become the most effetive race strategy until the 800m and above. For the 400m, I'd suggest going out within 1-1.5 seconds of your 200m PR and coming back in a 200m 2-2.5 seconds slower than your first 200m. If you're capable of running 23.5 in the 200m then I'd say if you're training is appropriate you should be able to run 51 seconds in the 400m.
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