All the advice about just being straight up with your dad is good.

Emphasize what you have learned. You've tried road biking. You've learned what it's about. You have a broader base of experience now (and it's true, you do!). You've learned a little about yourself, and now know yourself well enough to know that you want to go in a different direction.

Basically just be up front and emphasize the positive, because I promise you the experience you've gained so far is indeed a positive thing and you'll be surprised at how it might come in handy one day far into the future when you least expect it.