Opera star Katherine Jenkins admits she’s nervous as she begins her rehearsals for “Dancing With the Stars.” The Welsh-born beauty was announced last week as a contestant on the upcoming ABC season, and will be paired with dance pro Mark Ballas. She told us from London Friday, “I start my first day of training tomorrow, Mark is coming over here for a week of rehearsals. While I am so excited to be part of the show, I am a little nervous because I don’t yet know what I am letting myself in for.” While being used to the discipline of voice training, “This is something out of my comfort zone, it will be a challenge and hopefully something I will enjoy. I am ready for a challenge. I just hope I am physically up for the challenge.”