'American Idol' host Ryan Seacrest has joked he worries he will injure himself if he tries to keep up when his "athletic" girlfriend Julianne Hough is doing physical activities.The 'American Idol' host has been dating former dancer Julianne Hough for two years and while he is completely smitten, he admitted he sometimes worries he will injure himself if he tries to keep up when she is doing physical activities.He told chat show host Ellen DeGeneres: "She's such an athlete. I'm afraid sometimes to do stuff with her because I know I'm going to get injured."Ryan, 38, also admitted he has been thinking of marriage but insists Julianne, 25, will be the first person to know when he decides to pop the question.Â Asked when he is getting married, Ryan answered: "Good question. At the perfect time, and she will be the first to know."Julianne has previously admitted Ryan was her "first celebrity crush", but was hesitant to go out with him when they first met.She said: "I'm kind of dating my first celebrity crush, so I feel pretty fortunate."I was reluctant because he is such an influential guy and what if it didn't work out? Our first date, I was so nervous that I just sat there and interviewed him with questions like, 'Are you loyal?' and, 'Are you close to your family?'"