Piano Forum

You could have a listen to Gilels' version of the fantasy on youtube. His runs are amazing.

From what I remember when I play it...

Very slight delay in the first note or 2 for both runs.
Then fast as possible, but keeping a rhythm. I like slight wrist emphasis on the Cs as the first note of the smooth descending sequence.

Then make sure the last 3-4 notes are clearly defined, which I try not to slow down in the first run on the way down.
First run the notes back up start soft on the C, but slow down at the end giving emphasis to the F#-A-C-Eflat without making it louder.

I do slow down in the second run  (The G-F-E-D before the A) on the way down as well as the C#-D-D#-E before the F on the way up, making sure the F is very controlled and clear. Getting that run soft, clear and controlled at speed at the end I find quite hard.