Hey, Jillybean,
You most definitely are not cheating yourself by keeping cardio within a reasonable limit. It isn't so much a matter to total cardio time per week, but of total cardio time and intensity per session that can be resulting in glycogen depletion. For example, if you are doing 30 minutes of high intensity cardio three times a week, the effect on glycogen stores will be different than doing a single 90 minute session of high intensity cardio. The timing of your pre and post workout feedings will make a difference, too.
Especially if you are wanting to increase lean muscle mass, too much cardio can have a negative impact on your training. A glycogen depleted state means that the energy available for strength training won't be there, so you won't be able to put as much into it. Also, as blood sugar/glycogen levels fall below a certain level, the body will tap sources of stored protein (lean muscle) for building more glucose.
You can see why for improvement of body composition, glycogen depletion is something you'd want to avoid. Given that excessive cardio can lead to this, you will want to avoid that, too. So how much is too much? It will vary from person to person, but a good indicator of glycogen depletion or low blood sugar is the almost uncontrollable craving for sweets and white bread type products.
Based on what you mentioned in your post, I would say that the routine you are doing when you note a problem with cravings is probably too much cardio for you.
Take care and good luck!
Maribeth
