Please don't do half marathon without practice.
1. It may affect your heart and lungs, anyway nothing happen of that sort good.
2. About your knee joint its muscle spasm
dip both your knees in 4 min in Hot water
bucket and 3 min in cold water bucket with ice,
its Contrast bath therapy it will reduce pain.
3. If still pain is there take ibrufen or myoril
as muscle relaxant.