What are the pros and cons of adding some low intensity aerobic training immediately after a Simple & Sinister routine? Such using HR zones, Maffetone equations, or the talk test to keep it at Zone 2.
I would say the pros are all of the pros of low intensity steady state training. I don't think there's much in the way of cons, but if I really think about it, you are looking at:
1. A longer session - this starts to matter more if you have a tight schedule, or as your runs get longer.
2. Your aerobic activity is going to be maybe a little slower. I don't think this is a con per se, but it is something to be aware of if you are tracking improvements - your increased run fitness might be masked by the fatigue from your S&S work, especially if you are comparing your run to a previous run that was done after lighter kettlebell work. (You do a session with a 24kg, then run 3 miles vs. you do a kettlebell session with a 32kg and then run 3 miles - run improvements might be "hidden.")
3. Your aerobic activity might feel harder because you're more tired (perceived effort is higher than if you had done the exact same run without having sone S&S first). Again, this isn't necessarily a bad thing, but it goes hand in hand with #2 above regarding "fatigue masking fitness."
#2 and #3 can be "solved" by setting aside a day once a month or so and doing your aerobic activity fresh after an off day. This'll give you a chance to compare being fresh to being fresh and see if you're still improving. This isn't really necessary if you are including it as healthy activity, but it might be helpful to assess progress.
What activity are you thinking about including? What are your goals with including it?