Although there is a lot to understanding stamina for rolling, it's hard to understand how someone who runs and swims can be gassing.
Perhaps pushing your cardio training harder is the way to go, definately incorporate some hills and interval training into your running. A more varied cardio workout such as circuits, or just a variety of different activities will help target elements of your rolling stamina that running and swimming don't.
Circuits can easily be achieved by selecting a group of no equipment exercises such as burpees, squat thrusts, press/sit ups etc and swapping between body parts every couple of minutes, with only a thirty seconds rest.
Sit ups for three minutes, 30 second rest
Press ups for three minutes, 30sec rest
Dorsal raises for 3mins, 30sec rest
Burpee for 3mins, 30sec rest
Then back to another arm exercise such as hand step ups, incline/decline press up
then another abs exercise (there's millions of those to choose)
then another back exercise such as b52, hip raises
then another leg exercise such as step ups, squat thrusts, gun drills, squats etc
and so on until tired and sweaty.
All times can be adjusted to increase/decrease intensity, and you can go as slow or as hard as you like with each repitition.
Remember to warm up and stretch!