Seeking opinions on these two options. Im looking to drop some fat while retaining muscle <---isn't that everyones goal.

Anyhow, I heard that running to lose fat can be counter productive as running also tends to eat away at your hard earned muscle.

What im wondering is if hitting on a punching bag vigorously for 30-45mins would be a good alternative cardio workout to running? Any thoughts?

Anyone do a punching bag routine or know how many cals you burn while working out on a punching bag? Thanks.