Hello, Shena!
Planning ahead for your trip to Walt Disney World is a great idea! This will help everyone's nerves calm and will enable you all to really enjoy each day, simply because you are comfortable and confident in your plans...including how to get there. :-)
Take a look at this
map of Walt Disney World, so that you can get a better idea of where things are located in relation to each other. Find Epcot on the map, then look directly to the right. There, you'll see Disney's Old Key West Resort as well as Downtown Disney. (So, no, Guests can't walk from a park to DD.) Just across the lake from DD, however, is Disney's Saratoga Springs Resort & Spa, a resort which IS walkable to and from DD. Take a SS Resort bus from any park, and, once at SS, get off at the Congress Park bus stop. This stop, listed near the buildings labeled "F" in red, near the bottom right corner of this
map of Saratoga Springs Resort & Spa, will put you within a 10-minute walk to the Marketplace side of DD. The pedestrian trail marked on the bottom right corner of this map leads to the bus stop marked on the upper right corner of this
Downtown Disney map.
Once you've eaten, if you'd like to return to your home resort, simply hop onto your Resort Bus at DD. Easy!
Send more questions as you think of them; I'd love to help you plan your first trip!