i put on about 8 kilos when i first started flying.. but that was a combination of flying AND all the free food that was onboard at the time (you wont have that prob with JQ so dont worry... just bring your own lunch) The constant takeoff and landings (4 per day) are not the best on your body.. think of your organs like a sponge... everytime you go up.. they expand and contract.. so many people end up with minor bowel issues/bloating/weight gain - apparently as a doctor told me.. your organs are not designed to undertake the constant pressurisation... and will take a while to adjust.
Having said that.. lots of people have no problem at all.. so i wouldnt worry.
I used a pedometer for a while...and the amount walked does depend on your position and the time of day (fri night you do a million trips throught the cabin for more drinks obviously) but on average... we figured that you might walk about 4kms a day.
Only really painful if your not fit (so you wont have a problem) or you are wearing brand new shoes. *ouch*
most of us get to the gym reguarly with no problems..