Will the market be open on Sunday during the Marathon?

Yes

Yes it doesn't interrupt the marathon

I believe their policy is every Sunday rain or shine. On theor website they only announce closure for the holidays in December. Since its the 41st year of the marathon and the 105th year of the market, I doubt they missed anything. I believe Lower Waker is your access to the inner dowtown loop, or use 90.

It seems like they open every Sunday, except December 23 & 30. Their website is www.maxwellstreetmarket.us