I've been typing a fanficton recently so maybe that helped. I switched to dvorak keyboard layout because my hands started hurting with the qwerty layout and the internet said dvorak might help reduce repetitive strain injury.
My left wrist will still get sore and hurt if type more than 12,000-20,000 ish words in a day. Sometimes I really type like crazy all day when I start writing really bad fanfictions without a care in the world! It's fun.
but when i write stories i did so really slowly. i tend to 'feel up' the words and imagine the scenery one at a time or i would be too bored to continue on.