Majin,
Lenght of the ride depends on how many stops the shuttle makes on the way to the Marina. It could potentially stop at Challenge Park/Soak City, Sandcastle and Camper Village before the final stop at the Marina Gate.
Without stops it would take less than 10 minutes. With stops it could take as long as 20 minutes or so.
Walking on Perimeter road is a bad idea. There is very little room between the park fence and the bay. With drivers paying more attention to Millennium Force than the road, I wouldn't risk it. Take the shuttle.
For a price, the driver might be willing to forego the stops (wink, wink) though it is certainly not park policy to tip/bribe. Those drivers can be greedy though. :-)