Not directly about early Buddhism, but certainly still relevant, and related to early practices…
In the 7th century, the Chinese monk Yijing came to India interested in the Vinaya and practices related to it. In his book, he discusses in detail many of the habits and practices in India, often fussing about how monks in China don’t do everything the exact same way.
In any case, one of the very interesting things that he writes a bit about is exercise taken by monks and laymen in India, and their ideas about it. Some of it is probably very applicable to modern people who are now often living sedentary lifestyles (being sedentary is even mentioned by the author). The following comes from Takakusu’s translation from 1896, A Record of the Buddhist Religion as Practiced in India and the Malay Archipelago (A.D. 671-695), pp. 114-115:
ON THE ADVANTAGE OF PROPER EXERCISE TO HEALTH.
In India both priests and laymen are generally in the habit of taking walks, going backwards and forwards along a path, at suitable hours, and at their pleasure they avoid noisy places. Firstly, it cures diseases, and secondly, it helps to digest food. The walking-hours are in the forenoon (before eleven o’clock) and late in the afternoon. They either go away (for a walk) from their monasteries, or stroll quietly along the corridors. If any one neglects this exercise he will suffer from ill health, and often be troubled by a swelling of the legs, or of the stomach, a pain in the elbows or the shoulders. A phlegmatic complaint likewise is caused by sedentary habits. If any one, on the contrary, adopts this habit of walking he will keep his body well, and thereby improve his religious merit. Therefore there are cloisters (Kankrama) where the World-honoured used to walk, on the Vulture Peak, under the Bo-tree, in the Deer Park, at Ragagriha, and in other holy places. They are about two cubits wide, fourteen or fifteen cubits long, and two cubits high, built with bricks and on the surface of each are placed fourteen or fifteen figures of an open lotus-flower, made of lime, about two cubits (=three feet) in height, one foot in diameter, and marked (on the surface of each figure) with the footprint of the Sage. At each end of these walks stands a small Kaitya, equal to a man’s height, in which the holy image, i.e. the erect statue of Sakyamuni, is sometimes placed. When any one walks towards the right round a temple or a Kaitya, he does it for the sake of religious merit; therefore he must perform it with special reverence. But the exercise (I am now speaking of) is for the sake of taking air, and its object is to keep oneself in good health or to cure diseases.
I was familiar with the Buddhist view of walking as a form of meditation, and also walking around stupas and caityas to develop merit, but I was unaware of the important role that walking played as a general physical exercise in ancient India, or all the benefits ascribed to it.
According to anthropologists, our early ancestors developed bipedalism long before developing a larger brain capable of advanced modes of thought. In some way, our physiology is more fundamentally tied to walking than to the sort of thinking we would say distinguishes us from other animals.