Sub 60s 400

End each workout with 100m strides. Also do hills.

/thread.

what are your other relevant PRs such as 1500/1600 or 800 or 5k?

this will sort of change my answer, but heres my preliminary answer: run hills as much as possible, at any speed/effort. run strides every day, ending with the last one or two close to max speed.

hill sprints can help a lot, but make sure they aren't too long. something like a levee will be perfect, no more than 50 feet long

as for more distance types of speedwork, you could try out doing 200s every week. start with 4x200 with 5 minute recoveries at 95% effort, build to 6x200. do this once per week, tracking all of your times. if you have spikes, wear them. do this before your scheduled run after a 10 minute warmup and some dynamic stretching.

This depends on how much you've worked on speed until now.

Pretty sure I was not much better than 68 for 400 when I started running (I ran 2:24 for 800m that year so I know it was better than 72).

A year later I ran 59.7. A year later I ran 55.0

Strides three times a week.

Hill sprints sometimes.

200m intervals sometimes.

I would not start off sprinting in spikes until you've built some strength in your legs. Later, use the spikes.

60m acceleration gliders, mile pace accel to 95 pct and hold for 30m. Start w 2 min rest, work down to 1min. Start with 2.

Eventually add 60m with 5 min rest.

Other intervals, strides.