Squid, keep up the sprint work, that goes without saying, but you also need to add some strength work and metabolic conditioning work. As you know there is more to rugby than running.
Strength- deadlifts, squats, pull ups and presses, low rep, heavy weight is all you need for strength and conditioning. Obviously if you are new to weight training, seek advice on form and progress slowly.
Circuit Training- Weights,body weight exercises mixed with bursts of running will take your overall fitness to levels endurance running just cant touch. Short intense workouts that push you heart rate way up fast and engage as many muscles as possible are key.