Good job Juan!

Here are a few different ways you could construct this sentence

Since I was a teen, I liked run and I used to run everyday, first for relaxing after of studying and then for fun. Some years after, I joined to a athletic club and soon they came hard trainings and races.

Here is how you could say it in informal American slang

I've been runnin since I was a kid. I ran every day. Used to just to get my mind of my school work. Later I just did it because I liked it.  After a bit I started going to a gym. I ran my ass off, both in the gym and on the track.

I have always like running, ever since I was a teen. I used to run everyday. At first it was to help me relax after I studied, then I began doing it for fun.  Eventually, I joined an athletic club and trained hard and ran races.

Since my youth I have always enjoyed running. Used to it was only to relieve stress from my studies, but over time it became it's own reward.  When I grew a little older I got a gym membership and went on training hard and running fast.

Born to run

Since I was a teen, I liked to run and I used to run everyday, first for relaxing after of studying and then for fun. Some years afterlater, I joined to an athletic club and then soon therey came hard trainings and races.
In those years, I have ran some marathons, half marathons and my speciality, the mountain marathon.
Unfortunately I hurt my back while I did a workout while/st working out/whilst doing a workout, and I was forced to take a break from running.
I have been stopped now for two years, and now I think that is the moment it's time to leave my fears and start again.
Wish me luck!
Thanks.

Born to run

Since I was a teen I've always liked running, I used to run everyday. The main reason I ran so much was because running helped me to relax after periods of study, but I also found it fun. Some years later I joined an athletic club. The training became a lot harder and soon I started entering races.
During those years I ran half marathons, marathons and my speciality the mountain marathon. Unfortunately I hurt my back during a workout, and I was forced to take a break from running. I haven't ran in two years now, but I think the time has come for me to leave my fears behind and start again. 

Wish me luck!

Thanks.

The best of luck.