How do I make the roblox character gradually speed up overtime?

I am trying to make a Roblox Sonic fangame and I'm stuck on making the character speed up overtime. I've tried things like if the humanoid walk speed is bigger than 0 then wait .1 seconds and make the new walk speed 6, and so on. That didn't work. I have to animations, one animation for the start when the character is gradually getting faster, and an animation for the character at full speed. I want the full speed animation to play when the character is at the highest speed. This is for a StarterCharacter by the way. So does anyone know how to make it so you gradually get faster, and then at a certain speed a different animation plays? I've spent a long time trying to figure it out and I know it's possible because there are other Roblox Sonic games that do this. I'm really just beginning with scripting so I don't know much. If anyone could help I would appreciate it!