My doctor, who is very good, and who is constantly scrutinizing the evidence re concussion treatment, advised the following:

1. Stay sub-threshold of symptoms as much as possible

2. Exertion (physical or mental) is an index of recovery -- there is insufficient evidence to show that it promotes recovery

3. That said, some light exercise like walking is good if you can tolerate it and it doesn't exacerbate symptoms. When I asked to what extent I can safely push through symptoms, he said "a little" (emphasis on "little")

4. Once you can tolerate walking, you can try gradually moving up the ladder (for example, by trying light stationary cycling for brief periods)

5. Add running and/or swimming later in your recovery, as both involve more exertion and/or head movement / jarring

The process of adding physical activity can be awfully slow, but in my view there is no magic bullet -- you can't force it.