I'm sorry i'm not familiar with SharpMap but basically:
1) figure out what's the concept of 'vertex' in sharmap, then create a QuickGraph graph from it
AdjacencyGraph<SharpMapVertex, Edge<SharpMapVertex>> g = ...;
2) then find the way to load a map and populate the list of vertices
3) same things with edges
Once you have the graph, you can apply any algorithm on it.