Grad school for math can be a bit scary. If you've been out of math for that long, you'll need to do some review. All of the people I know in my graduate program are experts at at a large swathe of undergraduate topics: calculus, differential equations, linear algebra, combinatorics, probability, etc. And of course, they know even more about the topics they really like. If you have the chops to do it, though, math only gets more interesting from there: Real Analysis, Topology, Modern Algebra, Galois Theory, Knot Theory, Convex Geometry...