Typing speed can be a big issue. Also as for story telling, I've found that running a D&D campaign and having to react to characters who without fail want to roam around outside of the campaign manuels to be excellent training. As is telling stories to my children where they can be the characters. It started with little red riding hood, my oldest son only hadto hear the story a couple times before he wasn't going up the stairs to see grama but wanted me to finish telling him the story all the same. Then all original stories with him as the star.
Other than that reading thousands of sci-fi/fantasy books. Practice practice practice.
I spent about 8-10 years never getting more than 10k in a story down and was asked if english was my second language. Then my life was falling apart and I decided I would either write a book or stop the dream. I sat down and even though my story skipped between 1st person and 3rd person and it stank so bad my brother and I had to do multiple rewrites, I didn't stop. Its a heck of a lot easier to edit a bad story than it is to write one.
My advice don't give up, don't stop, don't go back and rewrite. Just type until its done. Then go back and spend a couple weeks fixing it up nice and pretty. You'd be amazed.
The Deposed King