In your platformer state machine, make 2 separate states for your walking speed and your running speed. Switch those states using a button press, and then change your platform behavior's speed/accel/decel/etc values during those states to match what you like.
EG
If playerState = walk
set acceleration to 800
set deceleration to 800
set max speed to 1200
On keyboard C pressed ->
Set playerState to run
If playerState = run
set acceleration to 1200
set deceleration to 1200
set max speed to 1800