The most challenging maps for you.

I won't play such maps anymore. There's no balance between difficulty and enjoyment. The game is somewhat reminiscent of slave labor on galleys.

To replay the same map multiple times for speed, without losing too much time, you can try playing on medium-sized maps.
Regarding enjoyment: personally, I find pleasure in the process of playing; the race for the best result "for speed" actually prevents me from feeling enjoyment. What does an athlete feel while running a race? Most likely, nothing, as all their energy is spent on developing speed!
So, maybe sometimes you should just play a map for fun?