A runny rice pudding means you haven't cooked it long enough. Continue cooking and it will thicken, I promise. If the pudding is too thick, you cooked it too long. Remember, rice pudding thickens quite a bit as it cools. It should still be a pourable consistence when it's done. If it's too thick, place it over medium-low heat and add milk and stir until it thins out a bit.