As a resource you can go back to several times, I highly recommend "Practical Programming" by Kilgore and Rippetoe. The first half of the book covers exercise theory, general adaptation syndrome, energy systems, and balancing recovery and stress. The bottom line is that the holy grail of losing fat while building muscle mass is something that is really only achievable by beginners. However, it does require some expounding here to understand why. But here are some applicable observations:
- Bulking is most effective at adding muscle when you start in a lean state.
- Walking is an effective method of active recovery, but not very effective for burning fat.
- There is more than one way to add muscle mass, depending on your goals.
Bulking is Most Effective When Already Lean
If you are already lean, you can tell by looking in a mirror whether the increase in weight is due to muscle mass or fat mass--or the relative percentage of each. The bottom line is that adding muscle mass is not easy. It can take a while to add a pound or two of muscle mass, but when you do, you will be burning significantly more Calories even at rest.
The majority of the problem is that your body likes to maintain the status quo. If it is in the mode of storing extra calories as fat, it will tend to continue storing more of your excess calories as fat than as (more accurately within) your muscle. If you are already obese (>20% BF for young men or >25% BF for older men), then your body is tuned to store fat. The best way to break that cycle is to get to 15% or below body fat (assumption: men). It's about that threshold that the body is more tuned to feed your muscles than your fat cells.
Walking Not Effective For Fat Loss
Quite frankly, walking doesn't burn a lot of calories. It does help, because it gets the blood flowing which helps reduce soreness and other issues with strength training. However, you need a way to get the best of both worlds. This is where we enter the world of conditioning. Conditioning can be as simple as adding a weight vest while you are out walking, or it can be as widely varied as the different forms of High Intensity Interval Training (HIIT).
In order to burn Calories you have to expend more energy than you take in--but if you are "bulking" you are taking in more calories than you burn. This is the conundrum, and yet another reason to separate the activities. The bottom line is that movement burns Calories. High intensity work will continue to burn fat just as low intensity work, but will also deplete your glycogen stores much quicker. Once the glycogen is burned up, the only energy source left is fat. But don't get too ahead of yourself here. See below when I bring in the concept of diet...
Multiple Ways To Train
If you care about increasing muscle mass, then training for muscle size is going to help your goals quicker and more effectively than training for strength. Paul Carter calls this mass building, or base building. It is similar to bodybuilding, but the end goal isn't the perfect ratio of muscle sizes. The end goal is to increase the ceiling of strength.
When training for mass (base building), I found myself increase in size around the chest, and decrease in size around the gut. That is until the holidays came and I started over eating my gains. My goal here was strictly eating to maintain weight. I wasn't doing conditioning, just long sets and a decent amount of volume. As long as I didn't eat too much, I was trading mass. However, once I hit equilibrium (i.e. still maintaining weight) the fat wouldn't come off anymore.
The other side of training for mass is that you are burning more calories because you are training more volume, which requires more work overall.
It's All About Energy Balance
At the end of the day, whether you are trying to get slim or build muscle, you have to balance the needs of your body for energy. You have to be in a caloric deficit to burn fat, plain and simple. You have to be in a caloric surplus to build muscle. Unfortunately, the capacity of the body to pull from the fat stores while building muscle is very limited.
Various diets attempt to address the conundrum by something called Calorie cycling. This gives your body more calories on the days you weight lift, and fewer calories on other days. The net result over the span of a week should be lower than your requirements if you want to primarily lose fat, or higher than your requirements if you want to primarily gain muscle.
As useful as Calorie cycling is, it can only do so much. In essence, it is best to pick one primary goal and pursue that while having the least amount of impact on the other goals. For example, if you are obese, the primary goal should be to lose fat while preserving the muscle you have. If you have normal body weight you can either maintain or go for muscle building while minimizing the amount of fat that inevitably will be added.