I think the problem is on the route as well. The last 12Km would be going down and up inside of Western cross harbour tunnel, Sheung Wan bridge, Central tunnel, Admiralty bridge and Causewaybay bridge . If those part not cause damage, the speed can maintain. Apparently, if the last 12Km is flat road, I sure I can do much better result in last two years.
I would not need the free bus service. My worse result is ~ 4:45 that happened two years ago.
It would assume you would not hit the wall in last 12Km when going up and down. There is no insurance. If you hit the wall last 12Km, you may not be able to make it. I reserve 5~10mins in case of hit the wall for recovery.