Aerobic exercise is largely about volume
It's about doing work for long enough to stress the aerobic energy system.
The aerobic energy system doesn't care what it is you're doing, it is only interested in pumping blood around, getting the oxygen to the cells and refuelling the ATP in the cells.
The modality you use to achieve this is almost irrelevant
While the "classic" aerobic exercises such as running, cycling etc are ideal.
You can also use circuit training, so long as you pay attention to your heart rate.
Aerobic specialist Phil Maffetone suggests that for maximum aerobic fitness ( or MAF, I'm always jealous of folk who make cool acronyms out of their names) you use a heart rate equal to 180-age
Then take a bit off if you're older, returning from a lay off or similar
Add a few on if you're under 25 or in tip top condition
Now keep this as your average heart rate for 30+ minutes
20 minutes will do, just about, 30 is better and there's no real upper limit outside of practicality
We can use circuits similar to the 100 Rep Warm Ups or those suggested in the WMD eBook (see shop page) just extend them out to 30+ minutes and watch your heart rate.
Other options would be to string together skill practice with rehab and mobility exercises for a lovely low key session that brings on a sweat but leaves you feeling ready
You can mix between running, shadow boxing, cycling, calisthenics using anything from 1 minute to 5+minutes per set.
Watching the heart rate of course.
This is also where so called "flows" fit in.
For example, yoga's Sun Salutation could be considered the original flow, several rounds of that done at an easy pace over 5 minutes.
Take that into 2 minutes of Hindu Squats, into 3 minutes of shadow boxing, a 1km jog and you have an interesting, simple circuit that done for 2-5 rounds will tick the box just nicely.
Regards
Dave Hedges