so the only reason you want to do cardio is training for your x-country carnival?
you have to think about this:
recomended bulking calorie intake per day
calories burned during exercises
recomended bulking calorie intake per day + calories burned during exercises = what you should be eating including the x-country training
best idea would be do have some liquid carbs after the workout, and eat a little bit extra to compensate for the calories burned (preferrably protein + carbs).
Im thinkign the best cardio would be just to use the x-country trainer. Do either a hill climb or random program and keep the resistance level high.
Doesnt matter how long the cardio session should be as long as you compensate for calorie burned. Try to keep your TOTAL workout session no longer than an hour and a half since cortisol (a hormone that breaks down protein) builds up greatly.