Experience is not necessary at this level: anyone who is in good health and fit enough to enjoy a good weekend hill walk can manage an A grade. Usually, easy treks are short in duration and at lower altitudes. However, walking always involves some exertion: trails are seldom flat, and you must still expect to have a reasonable amount of ascent and descent. You would normally walk for 5 to 6 hours during the day.
"An ecosystem is a unit of biological organisation interacting with the physical environment such that the flow of energy and mass leads to a characteristic tropic structure and material cycles." -Eugene Odum "the grandfather of ecology"