I just did the measurement - from the new headhouse, across the footbridge, around the shoreline, and under the bridge, it will be 0.87 miles to Assembly without ever touching a road. It's about 0.75 miles to Sullivan using much less friendly infrastructure. That's a bit farther than Harvard Stadium or Alumni Field are from the T stations that bear the name of the school, but a little closer than the latter is to Cleveland Circle and Reservoir. It's about twice as far as Fenway Park is from Fenway. Not dissimilar from the walking distance for Fenway to Hynes, which plenty of people do.
FWIW, it's also the same walking distance as Audi Field in DC is to the Navy Yard Metro stop. It's not the best, but it could be worse.