So, after the longest pedestrian bridge it only took a few tweaked searches to guide Me to the longest (walking friendly) trail tunnel in America.
The Snoqualmie Tunnel in Washington State is just that, at approximately two miles in length, this railroad track converted to a trail offers something unlike anything else. My reading has revealed descriptions similar to dark in the day; apparently I'll need a light.
I noticed on my most recent visit to the Iron Horse State Park website that the tunnels are closed right now, but scheduled to open again June 1st.

I like the general path I've found including these three points. A walk across America-

With the seasonality of the tunnel and my existing schedule, my mind now turned to when to take this walk.

Here I'll note that some cooperation will be necessary to see this through and without that help I can't make it happen, but as support begins to surface I'm thinking it will come down to can I really walk that far. Thanks to everyone that is already expressing appreciation and support for what We are doing.
It'll be exhilarating and hopefully I won't disappoint those who are investing in this emotionally.