You have several choices.....
1. walk to BW or BC and take the DTD bus from there.
2. take the SSR bus from Epcot to SSR, then walk or boat to DTD
3. take the POR bus from Epcot and then take the boat to DTD from POR
In all cases, I would leave at least an hour to get to DTD...in fact, I would probably leave 90 mins, just to be sure. You can always wander around DTD for a bit if you get there early. But, getting to DTD seems to take the longest amounts of time compared to all other destinations.