I have a post somewhere around here with tips...i've been EPing DD for 13 months - we're just about fully weaned to soy milk now, though. We took her to WDW when she was 8 months old.
My experience EPing @ WDW
My top tips for EPing at WDW
Specifically re: pumped milk in the parks - what we ended up doing is having DD drink whatever I last pumped. BM is good at 80-90 degree temps for 3-4 hours, and good at 69-69 degrees for 4-8. DD was eating every 2-3 hours or so, so when I would pump in the park, I'd just put the bottle in the diaper bag w/a cap on it for the next feeding. She didn't mind drinking room temp/air temp BM, so we didn't need to heat it this way. She doesn't like cold milk, and heating bottles from the cold milk became a big pain.
We did also bring a cooler bag into the parks. It's the cooler bag that came with my Ameda PY pump, so it had ice packs on all 3 sides. It stayed cold pretty much all day, though I did bring a second set of 3 ice packs that would fit it, in case we went back to the hotel mid-day and I could swap them out. I never needed to do that, though. 1 set was enough for the day. 2 sets allowed me to use the sets on alternating days, though, so they'd have enough time to fully freeze in the fridge freezer.
If you're talking about storing in the room, our room at SoG had a fridge, so I was able to use that for anything excess that I didn't bring/use in the park.
I hope this, and my linked posts, helps! If you have any other questions, I'm happy to answer if my experience can help at all!