One month, eh? Well, you don't have much hope for increasing your stamina by any reasonable measure in that type. You should starting getting your cardio up now for whatever event you end up in next. The best thing you can do is something specific to your sport; if you can find someone to roll around with you at a regular pace for 20 minutes straight, not counting warm-up or cool-down, that keeps your heart rate at the proper range (check below for the proper range), that would be the best. Otherwise, something that uses the muscles that you use the most. I'd figure a rowing machine would be good for you, or something that does a lot of knee extension, like a stairmaster or going backward on an elliptical trainer.
Proper Heart Rate Range (simple)
(220 - age) * 60-90%. I'd stay toward the high end, say 75-90%, but it's best not to do that more than every other training session. Do your other sessions at the 60-75% range. And get in as many days as you possibly can.
