Fantastic questions. First of all, I agree with your assessment of this forum. One other thing you won’t find here is negativity. When you say volume, I assume you mean in terms of training. However, you will also be increasing your total volume of blood, as well trained endurance runners generally carry a few pints more than your average pedestrians. This helps immensly in terms of carrying oxygen to muscles, removing waste and cooling your system. I do, howeverm believe your total volume can be maintained or even improved with a day of cross training. After a hard week of running, your system loses a lot of iron through sweat, urine and even through the pounding of your feet on the asphalt. It might be important for physical and mental recovery to spend an excercise session doing something other than running. You will benefit yourself by stimulating muscle groups that don’t get the attention when your main activity is running. I would listen to your trainer. I think you will be able to push your running workouts a little farther if you allow yourself a day of rest.