Yup, I quit cycling and sold my bike some months back, money all go to camera liao.
IS is always working regardless of focal length unless you switch it off. What it does is to give you 2 to 3 stops more of shutter speed, my 18-135 is reviewed and rated up to 4 stops.
Eg, you are at say 18mm, you will need to have at least 1/20s shutter speed right? with IS enabled, you can stop down to maybe 1/10s(1/15s, 1/13s, 1/10s), that's 3 extra stop of shutter speed. Now you will ask, why do I need to stop down since I can do 1/20s easily? Well, with the extra stops of shutter speed, you allow more light to enter the camera, hence you can have the option of not bumping up your ISO which will translate to less noisy pic.