Trying to recreate Sonic's running animation

Hi, i’m fairly new to unity, and i’m trying to make my own 2d Sonic game. But i’m facing a problem with his running animation.

His running animation (12 sprites) starts with Sprite 1, where he starts moving slowly, but to Sprite 10 to 12, it’s his max speed running.

But if i just make make the animation with Sprite 1 to 12, it will repeat everything, while i just want 10 to 12 to repeat once Sprites 1 to 9 are on screen.

Any ideas how can i do that?

Do multiple animations. So when you press your movement key, it uses sprite 1-9, and when you press the key for a longer time you make an animation transition to 10-12.