Edge contrast is crucial for realistic-type art. Some edges are hard, and some edges are soft, and you'll need both to create something more realistic.
Here, I've circled harder edges in red and softer edges in blue:
What I personally do is go in with a hard brush and blend it out until there is a softer edge. A good challenge, I think, is for every brush stroke you make, have a hard edge and a soft edge! It sounds fun to me at least lol
Fur practically depends on a contrast of hard and sharp edges, I've noticed.
Try to pick out the hard and soft edges in other pieces of realistic artwork and where! If you know the placement, you know where to start/work on!
This is what I have noticed in more realistic works, but I'm sure this applies to other styles as well as art is made with contrast!