When I stayed at the Boardwalk last time, there was one time when the Friendships were not running due to lightning. So keep that in mind. They may be forced to walk if the boats aren't running, or to wait until the storm passes. The walk was not nearly as bad as I had anticipated, but you do have to walk uphill a bit. I'm a slow walker and it took me about 15 minutes to get to the Boardwalk. Beach club is closer. With elderly folks, I wouldn't attempt it unless the boats are down. You want them to save their energy for the parks. So take a slower pace and enjoy the boat ride.
How far the walk is to the boat launch will depend on where your room is. When I stayed at the Boardwalk, I requested a room close to transportation, and that was a huge help. With the Beach club though, if they put you close to the lobby/boat launch, you'll have a longer walk from Epcot if the boats are down.