1. While you might be understood if you said "A is only 10 miles to B," it's better to say "it's only 10 miles to B from A," or "10 miles to B from here" if you happen to be in or around A at the time. However, the best way to say this is to include "away" and say something like "A is only ten miles away from B," or even "A is only ten miles away," or "it's only ten miles away." 2. Nope. You cannot omit the "to" in this case. "I've been Shanghai before" is confusing.