The best way to prevent muscle loss while being highly active is to eat. In your case, if you're doing lots of long term cardio, understanding that there's just about no way to keep from going catabolic and just about no way to keep your androgens from plummeting. This has been studied a bunch of times, and The Discovery Channel has a great series called The Human Machine that I encourage you to watch.
However, if you exercise some smarts you can preserve you muscle mass, to a degree, by MCT (medium chain triglycerides), complex carbohydrates, and good fats from poly and mono sources. Avoid saturated fats, if possible, and certainly, avoid anything fried. Sugar is NOT bad, especially post workout. You need those fast carbs to refuel your glycogen stores in the golden hour or so post workout.
You NEED calories in order to perform at a high level.
If you're looking to be lean and AND muscular, it's critical to maintain your resistance training AND to eat enough calories 600+ for every hour you're being active. If you're wanting very muscular and lean, you need to forego long hours of cardio and do HIIT instead, which will not deplete your muslce, and it'll improve your cardiac threshold more than about anything else. Ever run up bleachers? That's HIIT. You CANNOT engage in HIIT without a high level of fitness happening very quickly. HIIT will also help you avoid impact, and repetitive motion injuries.
When I compete, I bring my calories UP to support the increased activity level of my HIIT, and to allow my metabolism to kick into overdrive and to allow me to lean out. A fueled furnace burns hot.