The heart is a muscle. Heavy resistance training, ie strength training is known to increase muscle mass even in the heart.
Lungs can't be buff. They aren't muscle. While resistance training uses ATP for energy it also can create oxygen deficits during high reps which helps the body adapt to low oxygen levels. Long slow distance exercise like jogging uses oxidation for energy and helps the body adapt to more efficient use of oxygen.
The best training for cardio-respiratory fitness is strength training. Just because the fitness industry tied the word cardio to bikes and treadmills doesn't make it true. It's like all the bs about "core" training. Sheeple like buzz words.
One needs a mix of both. Lifting trains the body to use calories effeciently, more so than running or cycling. It does not make a well-trained weightlifter capable of running a fair distance. Just as most body builders suggest little "cardio", in order to use calories to build muscle, guys that run long distance avoid putting on pounds (even muscle), because it interferes with their goal. (running insane distances)
I like a mix of both. If I can't run a fair distance, how effective can I be as a rifleman? If I'm too weak to haul my gear, ammo, etc..a fair distance, how effective can I be?
Try controlling your breath and sight picture on a 20" wide target 400 yards away after running 3 or 4 miles. Carlos Hathcock employed running with gear, then dropping trainees on the line to get their hearts pumping and lungs wheezing before they tried to get focused and shoot. Lifting weights alone won't make you able to overcome that type of situation.