1st Gen Pokemon daycare help?

[WildPikachu]

So I'm playing pokemon red and I don't normally do this but I put my Mankey in the daycare because he was on level 14 when I caught him and my other three pokemon are on level 23.. when I play pokemon I always make sure every one is on the same level.

How does it work, steps or the longer you play the game?

 

[Astinus]

Each step taken in the game equals one experience point gained for your Pokemon.