Halong Bay is about 170 km (105 mi) east of Hanoi and takes about 3 hours to get there by road.
Duration: 3.30 hours
If you booked your junk through a travel agent, then transport there will be provided. Most transfers for the tour outfits in Hanoi depart Hanoi at 8am to 830am for Halong Bay, which is normally a 3.5 hour journey. Traffic in and around Hanoi is always very congested, and thats RUSH HOUR. Most Junks depart the pier at 1200-1230, so not sure how you can make that schedule, as even your arranged transfer can be running late for a host of reasons. If you are landing via a international flight, you have to clear Immigration first, so allow up to 1 hour for that.
The best way to get to Ha Long Bay is to rent a car from Hanoi. It costs approximately US$120 return. From Hanoi: 165km or 3 hrs 30 minutes drive.