what 100m speed is needed to run 55 sec 400m? as a distance runner

12 high. Maybe 13 if you have crazy endurance

My thoughts also. With the caveat that you're a 400m runner. Not a 100m runner trying to move up. You've got to have endurance to hold a certain 100m pace achieve your goal.

I've run a mid-54 4x400 split, and the fastest I've ever run in practice was 13.5-13.6. I did that 4 times, at the end of an easy run, in trainers (flying start though) and then I knew I'd be ready.

Not trying to take anything from your 54 split. But relay splits and flying start 100's are like comparing apples to oranges. Run an open 400 out of blocks and without the flying start you get on relays. Maybe you can still run a 54. But there's only one way to find out for sure. I really hope you can do it.

13 flat should be good enough for a pure distance runner.

Around 12.5s in training. Or around 13.00 in race from blocks

As a distance runner, why do you need a 100m predictor for 400m? You are even less likely to have trained for a 100m than 400m, so both are soft and poor predictors. I made it through high school and 30+ years beyond, and I have no real times (no races) for 100m, 200m, 400m/440y, and even 800m.

There's still time! Those old bastards at Penn seem to be having a blast.

13.75 seconds.