Actually, I started to get thoughts when I was 11 or 12, but I did not think about the reasons why much.  The whole thing with somebody who is 5-8 is more rare than anything else.  For quite a few people the first thoughts creep around puberty because that is when gender becomes more important, and the incongruity becomes more defined.    
I cannot wait to start HRT.  There is no fear about what will happen.  I am not sure how my interests will change...because they are already pretty feminine.  So how are they going to change even more.    
Of course my interests tend to skew a bit twee, especially with regards to music.  It is all pretty soft and quite, and more often just really cute.  The thing is I always wondered if I was attracted to the indie pop sub-culture because it did skew a bit more feminine in the first place.
The thing is I am not even on HRT, and I love chocolate, I listen to cute indie pop music, I love craft fairs and I am a sucker for any movie that can make me cry, especially old black and white ones.  
I play videogames on occasion, but pretty rarely.  I guess final fantasy or something on my DS.  
I keep asking myself, how is this going to make me more feminine in terms of interests.  The thing is with me, is this cutesy side will come out a bit more, and but the reality is I know it will help me be more comfortable wit myself, which is really what HRT is for.    I will have less fears about going out.    
I think HRT is a vehicle to help unlock your true self a bit more.  I personally have no anxiety about it other than what am I going to do at work when changes become really apparent.       
Don't worry about your interests changing.  That will happen throughout your life anyway with or without HRT.