500s speed workout

These "speedy" workouts are killing me. We did 6x500m with 300m walk between today. Is it normal that 6x500 much harder than 6x1k? I guess it kind of makes sense since I (am supposed to) run 500m faster than 1000m and because I do not have any speed in my legs, trying to run anything a little bit faster is a torture.

I did 1:43, 1:39, 1:41, 1:44, 1:41, 1:39 and I was dead.
Even if I had energy (and will) to try to run it faster, I could not. It seems that 18s for 100m is my limit and I can continue at that speed for a longish time but just cannot change pace.

I have always had hard time acquiring speed but once I got it, I could have run fast. But it takes me always a long time to get speedy and I lose it very easily.
I am sure my long runs do not help this issue a lot. Therefore I have decided to try to incorporate few pick ups into some of my long runs. That reminds me of good old Saturday bike path runs in PVD. Such good memories!
And our coach wants to do more speedy stuff with us, which means more suffering to me, but I guess it will help me in some ways.