Is this a correct usage of "來日方長"?
It's based on your intention. 来日方长 usually used for the two situations, if you want to tell sb that 


There's still time to do sth  
Need not to hurry in doing sth now


现代汉语词典（第五版）：未来的日还很长，表示事有可为，或劝人不必急于做某事。