As mentioned, the measured distance from the bottom of the steps at BWV to the IG is 0.36 miles. I never measured the distance from BCV, but from the image below it appears to be just a little over half the distance as from BWV to IG, so best guess is slightly over 0.18 miles, or probably about 350 yards (3.5 football fields).
That's from the lobby area of
BCV (not BC) . Be sure to take the shortcut and not go through the BC lobby which is a lot further.
For the shortcut, exit the BCV, go left, go under the canapy on the left, then go up along the side of the BC wing, and go out the small gate that connects to the walkway that goes around the lake, and goes to Epcot's I.G.
I presume once you get to the entrance your plan is to get a wheelchair or
ECV? If not, just walking around Epcot will be too much for her. The measured distance of one loop around the Epcot lake is exactly 1.0 mile.
(The red line is from BWV and the green line is approximately from BCV. Note that the satellite image was taken before BCV was built, so the green line is approximate location based on the existing BC building)
Hope this helps