Many years ago I went to see Mr. Dwight Hurlbut the founder and owner of Nutri-Foods, a health food store in Royal Oak, Michigan. Hurlbut had the habit of seeing people in his office to answer their health related questions. He was a very wise old man and he freely dispensed sage council to any and all who sought it out. At the time of my visit he was well into his nineties. I don't recall what ailment or question I had for him at that time but this is part of what he told me.
"Oxygen, you have to get outside and get oxygen."
Hurlbut, the wizened sage of metro Detroit clearly advised me to exercise outdoors in order to partake of the Creator's wonderful mixture of gasses.
So if you are a runner, cyclist, walker, rollerblader, or exercise newbie, do it outside. Breath in the air while you reap the benefits of movement. Trail running has the added benefit of getting you into the woods where the air is heady with aromas of wood, flowers, and all manner of organic matter. Wild life scurrying past will remind you that you share the world with other creatures.
Get out there and enjoy your world!