1. How many times a week can I workout ?
The prescribed program is 3 times per week. The first 5 or 6 weeks I started Simplefit, this was all I did. I do some type of exercise 6 days a week now, maybe some deadlifts on the off days and things like that, yoga. I take one of the weekend days off completely. If I'm pressed for time, I'll simply just do the Simplefit program at a minimum.
2. Where does cardio ( or "metcon" ) fit in ? Is it after the circuits or can it be on a seperate day ?
I'm sure other people can give you more specifics about high intensity and sprint intervals and such but on Days 1 and 3 in the workout, my heart is racing hard. I don't look for extra cardio but I do go out for a 6 mile run on the weekend for enjoyment. Other people follow the simplefit running program posted somewhere here. Fit it in whenever it works for you I'd say.
3. Could I work some core stability moves such as plank,bridge etc into the circuits ?
I do additional workouts and exercises now, after 15 weeks of the Simplefit Program, I do them after I do the Simplefit Program and on alternating days.
4. Both here and on crossfit 400m seems the most common running unit, why is this ?
Well, besides being one lap around a track, I think this type of training does not incorporate long endurance running/cardio as that really saps strength. The cardio comes from the high intensity aspect of the power movements and sprinting 400m as fast as possible. Sprinting benefits endurance running but endurance running does not benefit sprinting, so the theory is why run for endurance?
Apologies if some of this seems a bit dumb, but I'd really like to do this.
No need to apologize, this type of exercise program is quite different from the usual routines people have been taught. It's tought to transition from the standard workout routines everyone does to something like this. The main reason I stuck with Simplefit is results. I haven't always done it as prescrived but I adapt and modify my workouts to a style that works for me.