The key with training lies not so much in what you do, but in HOW you do it, how much do you do when do it, how often you do it, etc. There are many means of training to improve your speed, the planning is what will make you improve. This is the hardest factor for people to understand.
I urge you to use caution with what you read on the internet, and rethink your statement regarding plyometrics. Not they they are bad but you said that you “plan on working them a lot this spring/summer”
Plyos are only good when used at the right time for that right individual,
There is NOT a linear relationship between volume and improvement in anything related to training.
What position are you?
you say your speed endurance is poor, but does your position require speed endurance?
Furthermore, endurance is broad subject. Running 100-400s may not be the best case as endurance in the 400 is only specific to endurance in the 400.
Remember that just because something is good for running faster does not mean you have to add everything in at once.
I highly suggest you look into GPP essentials, CFTS, and Key Concepts to further your understanding.
Also check out some training logs here