If you enchant boots with Soul Speed, you will be able to run fast on soul sand. When you cross it, and return to normal blocks, you should have the normal speed, without extra speed.

However, if you run on soul sand and continue running on normal blocks, you will have extra speed for the next three blocks.

How to reproduce:
-Build a road in the air, made of soul sand, and end it in the air
-At the end of it, place three normal blocks (I used grass blocks)
-Enchant e pair of boots with Soul Speed
-Run from the beginning of the road to the end (where the grass blocks are), and don't stop
-You should have extra speed while on the grass blocks

For a type of normal blocks, you can use every block, except soul sand. The road doesn't need to be in the air, but it will be more easy not to accidentally run on other blocks.

In the screenshot you can see the end of it.