I am surprised... he always seemed to be very healthy and energetic... doing all kinds of things...
Honestly, when I saw the first Hobbit movie I was somewhat shocked by how much more aged and stiff he looked compared to Return of the King. It was clear that age was getting to him, although I wasn't aware of any serious medical issues.
On a side note: A lot of people talk about his time in the special services, but let's take a look for a second at the other stuff he went through:
- Suffered failure of his optic nerve during training, preventing him from being a pilot (his first choice of service).
- Dragged himself out of depression and decided to join the intelligence corps instead.
- Nearly died when his airfield was bombed.
- Caught malaria and recovered six times. Six. Times.
- More or less singlehandedly put down a mutiny with his knowledge of Russia.
- Went to Italy and saw some of the worst fighting there, including Monte Cassino.
- Nearly died again when a plane carrying armed bombs crashed on takeoff.
- Went to Rome, where he met one of the leaders of the Italian Resistance... who just happened to be his second cousin.