What is a simple plan to break 17 minutes in the 5k?

Forgot to mention, my PB in the 5k is 18:30 from 5 months ago, although I think this is a bit soft.

I've just completed a marathon in 2:56. My training during the marathon training block (e.g. 1k / 1mile reps) suggests I am in a sub-18 shape currently.

That’s not far off from what I’d do.

something like

Tuesday: 400s @1:15 with one lap jogging rest until reasonable fatigue. Might be 5-6 at the beginning of the training block, should develop to 12-16 by the end. (If you can put up 4 miles at <5:00 pace your body will be ready to handle 3.1 constant miles at 5:2x pace just fine)

Thursday: either 20-30 minute tempo run, 40-60 minute steady state (think 20-25 seconds a mile slower than tempo pace, 800s or 1ks at a bit faster than tempo pace with 400m or 800m jogging rest.

Saturday/Sunday long run, vary between an easier and harder effort each week. Easy long runs serve an important purpose to develop your capillary efficiency.

5 x 1000 in 3.24 with zero recovery and try and do the last rep in 3.23

I would scrap the 400s for the 1000s (or something similar) you mention above and also do a 3 x mile workout with no > 2 mins rest @ or as close to RP as possible. Too much rest on the 400s for any benefit. Don't stress if you bonk on the 3 x mile workout; it's a killer.

Space the tempo further than 1 day from your track workouts and keep them medium-hard but controlled. You can run 1 hard every 3 weks or so if you're feeling saucy. The long run is great, but no need for anything > 12 miles.

When you get closer to your race date (4 weeks or so), do 8-10 X 400 a few seconds faster than RP with 1 min rest. You can also do variations of this at 600 meters with a bit more rest.

Strides and 300s also helpful whenever you are feeling it. That should be enough to get your legs used to the faster turnover.

Seems sensible enough. I needed 75 mpw to break 17. But you may be more talented and able to do it on less mileage. I never did anything faster than 5k pace in training so you might be okay.

One possible suggestion, add 6-10 80-120m strides after 2 easy runs per week. Focus on form. It's not about speed. It's about running them as fast as you can with good relaxed form which is around 800-1500m pace for most people. Maybe jog lightly in between them for 30 seconds or so.

Since you have run a marathon in 2:56, I suspect that you may be limited by speed more than endurance. I would train for a few months more like mid-distance runner with the focus on breaking 4:50 in the mile, while still keeping the mileage on the 40-50 MPW; after that, switch to more 5K specific training and after a month or two the sub 17:00 should follow.

You didn't indicate what pace you are running your Tues workouts. You need to make sure you are incorporating race pace (or near race pace) in your workouts if you want to be comfortable and efficient at that pace in the race. I also think for 5k that it helps to do some work at faster than race pace.

For Daniels plans, you can simplify by just doing the top priority workout for the week. For example, in his 5k plans, phase 2 top priority is R-pace and phase 3 is I-pace. Or when you get a better feel for your strengths and weaknesses, you can fine-tune which workouts you do at different points in your training. For example, if you struggle with his I-pace workouts, then you need to do more work at that pace/intensity.