I would suggest that you work on the coastline first. At the moment it looks unnaturally smooth and follows the border of your sheet too much. I did the same thing with my first map, too. You should reduce the landmass and give it a more natural shape, epecially if the scale is that big.

Once the coastline has a natural flow to it, it's easier to work out the mountains, since it gives you a better idea of how the tectonics might work. From there you can work on the rivers.

I always like to have a look at Google Earth to get a grip on certain landmass-shapes or terrain features.