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.