There is a Super WalMart really close to MCO on the East Osceola Parkway(only 12miles away). There are other grocey stores listed on Deb Wills site that include addresses and store hours. Click on the Beyond the Parks tab and on the left hand side, there is a menu and Grocery Stores will be under the heading of Orlando Area Information.
There is an Albertsons just off the Greenway at the Orange Blossom Trail exit. It is not 24 hours but it is open til 1:00am. This would be a quick exit off the Greenway (it is straight across at the end of the off ramp) and back on again for the remainder of the way.