Planes are incredibly tight on space, particularly outside of first class. 1) Is she going to repeatedly climb over the people near her to get up and exercise? 2) The aisles are usually occupied by the flight attendants and the food cart. 3) Do you want someone in the aisle beside your seat exercising? I don't. 4) Ever heard of air rage? Given passenger anxiety since 9/11, I don't think I'd want to be doing exerises, irritating other passengers, and making them think I had a screw loose.
Okay, I'd missed that part. Three hours is not an unreasonable length of time to be seated. Many international flights are much longer, and it's probably a good idea to split any flight of four hours or more in duration into two sitting stretches with a walking-around bit in-between. Probably the easiest way to accomplish this latter bit is simply to get up and walk to the lavatory whether or not one needs to actually use the facilities. Since there's nothing unusual about using the lavatory in flight, such action should demand no particular attention.