You still have several rivers splitting and/or traveling from ocean to ocean here. It might help if you think of your landmass as an irregular dome. If it's so flat that a river can go all the way across it, then it would erode into 2 islands instead. So, your rivers will start at the highest point and flow downwards toward your major seas/rivers. They join on their way there like the branches of a tree join on the way down.