I have been going to the gym maybe 4 times a week for the last year. Was on ww before I moved back onto atkins.
I notice a big difference with the amount of cardio I can do now on atkins. Before I could do an hour straight easily, now I have to do 15 mins, weights, 15 mins, more weights etc to get my heart rate back down again and have a little rest.
I did read somewhere the sciencey stuff as to why its harder now.. something to do with carbs are used for immediate energy... glycogen... muscle fuel.. something something.. I will go try and find it.
Sukie does a LOT of running, and Nex does all kinds of stuff so they might be able to give you advice too.
If you feel fine when doing cardio then carry on as normal, but just be aware that your body is working differently and stop if you feel dizzy etc.
The main thing you wanna do I guess is preserve muscle mass as you will always lose this when you lose fat. I think the ratio is something like 25% of weight lost is muscle if you dont exercise.
I do 45ish mins cardio, plus all the weight machines when I go.