Brevity is the soul of wit. If you can write well, you shouldn't be trying to write longer.
In any case, if you want to write more, begin describing every sense more often. The touch of skin, the texture of fur, the warm, humid exhales of your character. Your characters smell, the sounds of your character breathing in and out, the sounds of your character making the mattress creak as he moves.
As for writing faster, practice imagining things in depth. Maybe read more. Maybe practice typing at
https://play.typeracer.com/