You can't walk from Poly to MK. You can walk to TTC and get a monorail to Epcot until 10 p.m.
After 10, it's taxi to the resort or bus to DTD, unless Epcot or DHS is having an EMH night, in which case you could take a bus to there.
Taxi fare from Poly to ASMo will probably be in the $20 range.