It is mostly seen, while we workout in gym, we lose lot of time waiting for either treadmill or weights. While working out with weights especially the break between sets gets prolonged and it suffice the benefits of conditioning while working on sub-maximal loads. Further discussing more on conditioning, its a workout which is a high-intensity interval based workout characterized by short periods of all out effort followed by periods of rest (short time).
Over time as your workout, work capacity will increase, meaning you will have the ability to accomplish more work in the less time. You’ll notice that there is no prescribed number of reps for this workout – you should simply aim to complete more reps in the same amount of time or add more weight as you progress and your conditioning improves.
Imagine doing 20 reps in 4 mins which describes less conditioning body verses 20 reps in 2 mins which is more conditioned to take load and also produces less metabolites.
Simply performing the kettlebell swings elevated and maintained the subjects heart rate and V?o2max at an average of 85% and 65% of their maximum. According to researchers, The American College of Sports Medicine recommends an optimal intensity of 60-85% V?o2max to improve and athletes cardio-respiratory fitness.
Either you are professional power lifter or marathon runner or even professional body builder, working on conditioning programs is imperative to improve performance for your lifting or running.
Kettlebells is unique tool which not only helps to improve conditioning but the compound movements done with Kettlebells also helps in overall strength gain and testosterone development. See typical sample workout with Kettlebells and body weight.

4min kettlebell workout for conditioning

  • 2 swing, 1 clean, 1 swing, 1 clean (repeat for 1 min) left and right hand for 2 mins
  • 2 swing + 2 squats (repeat for 1 min) left and right hand for 2 mins

4 Min Body weight training workouts for conditioning

  • 20 push ups , 20 hindu squats, 10 burpees
  • 20 side sit ups, 20 walking lunges, 20 hindu push ups

repeat after break of 30 sec if needed
Further Kettlebell lifts such as snatch, jerk, clean and jerk, double jerk and many other lifts are of course for people who have high level of fitness or are athletes, however simple kettlebell lifts as mentioned can become important part of lifting in gyms or even doing easily at home.
Professional body builders or power lifters who have reached plateau of workouts, for example heavy dead-lifts. Kettlebell workout like swing or snatch (done with correct form and technique) have seen significantly improving the lifting in short time, Kettlebells also compelement in effective use of hip flexors and lock outs which is important in power lifting.
Swinging Kettlebells with proper hip thrust and generating force gives a cross over effect for dead-lifts in many cases.
Snatch on other hand employs the valsalva pressurization techique during each repetition. So, not only do you stimulate an expansion of heart wall (called eccentric hypretrophy), and there by increase in maximal oxygen uptake (VO2max), but you also stimulate an increase in thickness of heart wall (called concentric hypertrophy).
My urge is to imply Kettlebell training in your fitness programs 3 times a week, on your weight training days and of course you have to run everyday!