detrimental workout???

could running 6x400s at 80-85% hurt my cns, strength, form, endurance, or speed??? b/c i felt really really fast before that workout. the next day, i couldn’t do my drills really quick anymore cuz i felt physically exhausted and mentally drained. it felt like someone had robbed me of my speed. i also felt very slow the next day b/c when i did my baton exchanges, it felt like i was running in slow motion. also after the workouts, i had to run 2 or 3 400m races during the following weeks b/c my coach said it would help my 200m endurance. mind you, i am horrible at the 400m. it’s been 4-5 weeks now since that workout and i have not PRed in the 100m since and i improved in the 200m by only .07sec. could that have hurt me in one of the categories listed above. could you explain the theory behind all this nonsense so i can avoid it next time???