My hands both tend to swell during long walks. what could cause this? and can i do anything about it?

Gravity: and vaso dilitation may cause hands to swell especially if its hot after exercise use cool compresses to reverse edema

Few possibilities: With both hands swelling, unlikely to have a blood clot in both arms, but you are on birth control, so that is possible. It may be related to your thyroid condition or some other autoimmune process like a type of Raynaud's phenomea. I would make an appointment with your doctor to do an exam.