I have a mining portal hidden in the end under the ground. When you spawn on the floating obsidian blocks fly toward the main End island and you should see a hole in the wall where I dug into the side. Make note on where it is, then you can gather dragon breath and leave through the portal. The portal sends you many thousands of blocks away from our city portals though so you'll have to make a trip. It might be helpful to mark on your map where our main city portals are in the mining world before attempting this.