There is massive amount of workout information freely available and as suggested you should certainly familiarise yourself with the work of Clyde Hart as one of, if not the most, succssful 400m coaches in the world.
I’ll mention here that one of the training decisions you need to make is whether to go from “short to long” or “long to short” and Hart is of the latter school working from greater volume and slower speed rather than developing speed first then building the endurance to maintain it.
One possible consideration in using a volume approach with regard to the 400m is the finding that it is 60/40 anaerobic/aerobic and hence there is reason to ensure that the aerobic system is fully developed.
You also do need to give some thought to how to progress and vary your sessions over the year so that you do not burn out with too many high intensity sessions too early in the year. You do not need to constantly test yourself to destruction.Think about building your capacity over the long term rather than your ability to endure the hardest sessions for the sake of it.
With that in mind a fairly conventional approach to track sessions would include:
- Acceleration runs over 30-40m e.g 4 x 30m from blocks bulding up from 4 x 15-20m
- Speed over 60-100m e.g 6-8 x 80-100m fast but relaxed with full recovery
- Speed Endurance over 150-300m e.g 6 x 150m or 6-8 x 200m or 3 x 300m (about 40 seconds effort)
The above would follow a standard sprint warm up.
You can combine acceleration and speed on one day with acceleration and speed endurance the next with the speed endurance sessions varying between shorter reps on one day and longer reps the next but generally I would not do more than two speed endurance sessions a week.
Your speed endurance might for example develop over the following lines for the shorter reps:
Early in the season 10 x 120m runs at 400m pace with 5 min rest progressing to rests of 2-3 min. As your condition improves increase to 6 x 150m with 5 min rest progressing to 2 min at the same pace. Finally progress to 5-6 x 200m run at race pace.
The above needs to be balanced I think with easier longer runs for aerobic conditioning and recovery. Running cross country or hills is good conditioning and a hill seesion can be incorporated into your weekly routine in early season.
Feel free to use or disregard all the above. The main thing is to inform yourself, consider the information provided and try to devise a plan in which you believe and which is appropriate for your aims and circumstances.