Could you correct me, please?

Dear friend,

I am writing to you because I want to tell you my decision to participate in a triathlon, and I would like your opinion about this.

A competition of triathlon consists of three sports: cycling, swimming, and running. The distances depend on the overall difficulty of the triathlon.  (Really? I thought these distances were standardized.)

I want to try the triathlon because I love to do sports. So far, I have done marathons and mountain bike races and this is a little dream that I have always had.

I know that its a very difficult competition, but my goal will be finish it!

I have already started with my training.  I work out every day before work. (You can also say "I exercise every day before work," but this sounds strange.  (Se oye mal.))   Every day I run for an hour and go to the pool. I also ride my bicycle five days a week. <---- Do you mean that you train with your bike every weekday or that you ride your bike to work? (ir al trabajo) 

I have changed my diet, and I only eat healthy food.

I look forward to your answer!