It's all relative, you need to ask how fast you'll be going in realtion to another object! I mean you'll be going about 8 MPH in the environment of the bus but if you expand it to the earth it will be something like 63MPH. (or 47, depends if you are running towards the front or the back!) Then if you expand it to relative to the sun its about 67,000 MPH give or take a few hundred, then expand again to our galaxy it's around 600,000 (not taking into account the spin of the earth around the solar system and what not)
The real question though is why would you be running on a bus?