I am currently reading Biochemistry for Sport and Exercise Metabolism, and there is an interesting chart that shows how if your exercise pace is low (e.g. walking), human body is primarily using body fat as a source of energy and glycogen storage is basically kept intact. Since I am currently in the process of building muscle mass, theoretically this seems like a reasonable way to burn excess fat without impacting my muscle gains. Would this work? Also, what would be the best way to approach this, should I go for a walk 2 hours after a meal so my blood sugar is down and then walk for 1-2 hours?
Unfortunately the answer to your question seems to be: NO. Layne Norton knows more about this topic than most and here is an article he wrote about it: BEST FORM OF CARDIO FOR BODYBUILDING? As you can see walking falls into the same category as endurance running. Interestingly cycling seems to be far better than walking or running.
It seems the only cardio that does not impact muscle mass is HIIT eg. sprinting. The sledge push or burpees can be done as a HIIT exercises and are gentle on the joints compared to sprinting.
I train strength and walk for at least 30 minutes every day. It is a trade-off. Walking may not be the best for muscle gains but is excellent for general health.
Schwarzenegger even quotes that walking is absolutely fine from his book as well as many in the medical world. I'd trust health experts over bodybuilders or random fitness articles as walking is absolutely fine if you keep it moderated and don't walk more than an hour at a time...
Walking at a moderate to high pace(at least 2.5mph) after ten minutes will start to burn pure fat. Walking more than 60 minutes and your body will start to use carbs. The higher intensity the more carbs/glycogen your body uses. HIIT burns a lot of carbs, but your body recovering later will burn fat in EPOC.
According to Schwarzenegger, our bodies were made to walk long distances everyday, due to hunter/gatherer instincts or to conserve energy to fight. There is no harm in walking, and you can walk frequently.. it's so low intensity that it doesn't really affect you.
The argument is if it's efficient. Walking burns about ~100 calories an hour so to lose a pound of fat that's 35 walks you have to take. It's great to walk as a hobby or at lunchtime at work but it's not going to be your biggest motivator.
Walking-low intensity, burns pure fat(not most efficient way, but you can walk as much as you want)
Jogging-moderate intensity, burns mix of fat and carbs(best for heart health but not fat loss)
Running, jump roping,etc-high intensity, burns carbs.
HIIT-burns carbs, burns fat in EPOC.
Combine different cardio intensities and exercises from above to get best results coupled with weightlifting and a calorie deficit. You can either exercise less and eat less, or exercise more and eat more. "Burn the fat, or starve the fat" trust the medical experts that say walking won't affect muscle mass.