Trying to cancel login to a realm and then entering a single player world will eventually log you into the realm at the coordinates your player was at in the single player world.

Steps:

1. Attempt to join Realm
2. On loading screen, press Cancel
3. Immediately join single player world

At this point you should be in the single player world and then get placed into the Realm at different coordinates than where you left off.

In limited testing, I found these coordinates closely matched the coordinates the player was in in the single player world.