Long distance relationships rarely if ever work out. I don't think you were cheating, you just fulfilled your needs for companionship. I was in a long distance relationship once and that woman was jealous of anyone I talked to on a different site. Even though she was married. It was a no where relationship. If I were you, I wouldn't say a thing.
My long distance boyfriend and I met in April. I came back to North America and we kept talking. In June, I went back to him and told him that I loved him. But in May, I was physically intimate with someone else. Was it cheating? Should I tell him?
Yes it could be legally construed as cheating depending on your relationship level here but if only once and you don't want to lose this other person then forget it ever happened as if you tell him, its over for sure so its up to you here. Good luck