Interesting idea! I would be careful with the textures you use. The current one looks like dried up and cracking mud which makes the landmass feel pretty small. I would try to sample textures from real places instead. The inside of the continent is more likely to be dry and the outside bits wetter I would guess. I would start with tectonic boundaries and plate directions to determine mountain ranges and such from that to determine your drainage basins. Since you are going the mythological route I guess you could do whatever you want geologically.