Are you familiar with DisneyBound (www.disneybound.co)? The idea is regular clothing inspired by characters - for example, one year I did an orange shirt and striped dress for nemo (I couldn't find orange and white stripes). That's what I usually do - take the colors of the characters and wear running clothing in those colors.
I'm also a big fan of RawThreads, who make some cute character shirts, but they're part cotton and don't work well for everyone (but I ran the marathon in one of their shirts last year and was fine).
In terms of Pop to Epcot ... we're probably gonna be on one of the earlier buses, but I like getting character pictures pre-race. The event guide (whenever it comes out) will have a recommended last time for you to be on a bus.