I said to go exactly 2 56.00 is bang on the limit. You are just guessing that you could not beat you hs time in college.
As you mentioned Horwill his 4 second rule is the limit. Most cannot manage that & most people would say you need faster to go sub 2.
I've seen runners with a 400m on pb of 59 split an 800m in 58. Most runners don't know how to pace a 400m in lane tend to go too quick & die.
It's impossible not to gain in strength from 16/7 to 21 you don't need to be musclebound.
Relay is always last on the program. I bet you'd already run when you did your 400m relay or.
Either way imagine if you run for years but only run a couple of 5km races. You are not going to find your capability off that.
Still if you insist you didn't improve by 1.5 seconds in 4 years so be it!
Are you s reverse of all those ultra runners that like to trim their pr''s!