You need two animations, while accelerating set the jogging(?) animation (the non full speed one), you can even increase the animation speed in function of the player speed to accelerate the animation along the player.
When the player reaches a high speed, set the running animation (the full speed one), and switch to the lower speed animation again if the player speed is reduced.
PS: If you want to make a full Sonic game you’ll face problems with the loops 