". When we go for a walk, the heart pumps faster, circulating more blood and oxygen not just to the muscles but to all the organs—including the brain. Many experiments have shown that after or during exercise, even very mild exertion, people perform better on tests of memory and attention. Walking on a regular basis also promotes new connections between brain cells, staves off the usual withering of brain tissue that…
Unless you live on a mountain-top or near minefields, walking is still very easy compared to most other options except sitting and laying on a bed.
And even if you do live in such a place, it's usually pretty trivial to find a non-mountainous/non-obstacled path to walk in the surrounding area. Didn't say it has to be just next to your door.