I would use a separate variable called "my_speed" which is used for the current speed the player has.
When it collects powerups you can change this variable without messing up the other ones like walk speed, run speed, super mode speed etc.
So you can simply make a collision event with obj_adrenaline and do:
GML:
my_speed += 2;
alarm[0] = 60 * 15;
Then in the alarm[0] do:
And if ever you need to set the my_speed back to standard walk speed you can just do my_speed = walk_speed.
Edit- I just realized this is exactly what woods said apart from the separate my_speed variable thing! The code is exactly the same. Sorry
@woods I got carried away, I just love coding so much