This is quite a common scenario for those who have recently joined the RM. You experience weight loss and then gradually put the weight back on as your body puts on additional muscle to cope with the additional exercise that the job entails.Interestingly posties who become drivers, PHG or are off for extented periods are prone to put on considerable amounts of weight.
I've measured the amount of calories that I burn on mixed walk ( front doors, flats , tower blocks) over a couple of weeks and the average calorific burn was about 300 calories an hour. This means that you are burning approx a 1000 calories on a 3.5 hour walk, this on top of your body needs just to survive approx 1665 calories (based on your height and weight given). In order to gain weight you will need to eat/drink at least 3000 calories a day to put on any meaningful amount of weight. ( please make sure this is a balanced not just crap!)
If you are worried about your weight loss you should go to your doctor and talk about it.
Especially if you have found yourself having bouts of diarrhoea or constipation. Otherwise enjoy it!. You'll probably never be fitter and no-one loves fatty ( not really !)