I like to swing kettle bells when I strength train. I don't swing kettle bells during every workout, but I do try to incorporate kettle bell swings somewhat regularly. I like the fact that the basic swings engage a lot of muscles at one time. Because I am an endurance athlete, I don't need to swing kettle bells to get a cardio workout. Rather, I use them as another way of building power. However, other people do like to use kettle bells as a way of stressing both their muscles and their cardiovascular system at the same time.
One problem. When people multitask, they usually end up making more errors and completing their work less efficiently than those who focus on one task at a time (see here). Similarly, when working out, to best improve muscle strength, it's best to exercise one specific muscle, to focus on the one area (or system) that you want to get stronger. At least that's what the most recent research, as reported by Alex Hutchinson, seems to suggest (click here).
I highly recommend kettle bell training as a part of any comprehensive fitness program, and by working out with kettle bells you can improve both your strength and your aerobic fitness. However, one should not rely solely on kettle bells if maximum strength gains and maximum cardiovascular fitness improvements are your goal.
Of course, these kettle bell studies are consistent with recent studies that find that adding instability to an exercise, in order to engage more the core muscles, is not the best way to build maximum power and muscle size (see here) in the non-core muscles being worked.
In other research, it looks like one bout of muscle soreness helps muscles adapt to future stress (see here). This research emphasizes the importance of training on courses similar to the one you will race on.
Today I completed a fartlek run of 60 minutes, including six pick-ups of anywhere from 50 to 84 seconds. I ran the pick-ups hard--mile race pace or faster, and made sure that I was fully recovered before doing another one. I finished the day with one short hill sprint. I enjoyed the less formal nature of this workout.