Hi Rachel,
I hope you have a wonderful first trip!
The Disney Transportation system, including the buses, runs one hour prior to park opening and one hour after park closing. It is the same for Downtown Disney as well. The buses also do not run between resorts unless the buses are on the same loop such as Port Orleans Riverside and French Quarter.
To get to either the Boardwalk or the Grand Floridian. You will need to take a bus to a park and then take another mode of transportation to your final destination. For the Boardwalk, you will head to Disney's Hollywood Studio and then take a bus or walk to the Boardwalk. For the Grand Floridian, you will take a bus to Magic Kingdom and the a boat or monorail to the Grand Floridian.
You could visit the Boardwalk when you are at Epcot via the International Gateway and the Grand Floridian when you are at Magic Kingdom. You could plan a meal at both.
However, if you are going to stick to your original plan, you will need to head to the Boardwalk first via Disney's Hollywood Studios. Then from the Boardwalk take a bus to Magic Kingdom and continue to the Grand Floridian. From the Grand Floridian, you take a bus to Downtown Disney. While I love the Boardwalk, there is not much to do there if you are not eating.
Wishing you a magical vacation!
Beth