Moving problem

I’m having trouble programming the walking sequence for my character. This is what I want:

When you press the down key, he moves down 48 pixels, but only one pixel at a time, so even if you release the down key before he moves all 48 pixels, he keeps moving until he’s gone the entire length.

This is because I’m making a tile based game, and I don’t want him to be able to stop in between tiles (my tiles are 48 x 48 pixels).

Can anyone explain how to do this? I tried using a for loop, but I’m not good at those and it didn’t work out.