TREKKING in Japan can be demanding. Some of the best walks, close to Tokyo, involve climbing peaks more than 3,000 metres high. The physical challenge makes the journey worthwhile and ultimately leads to a new sense of fulfilment. It is literally possibleto walk stresses and tensions away. More than 70 per cent of Japan's total land mass is mountainous. Walking along the extensive network of trails can be an invigorating experience, offering travellers a complete change from schedules, crowds, pollution and other features of modern urban living. The first-time visitor may be surprised by how clean the mountains are. Water from streams can be drunk freely, the trails are free of litter, and the air is crisp and clean. One trek on the list being offered by Carpe Diem Expeditions Limited, Hong Kong, is to the Southern Japanese Alps, less than two hours by train from Tokyo in an area called ''Ho-o-sanzan'' - the three Phoenix Peaks. Historically, this area has a spiritual significance. Childless women made pilgrimages to the tallest peak to pray for children. At its highest point the trail reaches an altitude of 2,900 m. In winter the area is snowbound, and even in summer drifts of snow are common. The trail is well-marked and easy to follow, winding through dense evergreen and deciduous forests. The seasons are clearly defined here. Spring comes late, but by June wild rhododendrons and azalea are in bloom, and in autumn, maple and birch transform the green lushness of the mountains with vivid colours. Walking parties stay in mountain huts, which are simple in construction and in furnishing, but comfortable resting places. The caretakers are often atypical Japanese who have given up city life for the peace and quiet of the mountains. They prepare all meals in traditional Japanese style. At the start and end of the trek, walking parties stay at hot springs inns. This luxury will keep even the most weary trekker going. A walk in this area can be comfortably completed in eight days, door-to-door from Hong Kong. The transition from Hong Kong to the mountains is short, and for the busy executive a trip of this duration does not mean giving up too much precious holiday time. Carpe Diem provides guides with extensive experience of walking and know the mountains well. The mountain huts have basic amenities. Futons and blankets are provided to sleep on, so there is no need to carry sleeping bags or tents. The treks are within the capability of the moderately fit, and provide an excellent opportunity to boost your level of fitness in a natural and gradual manner. The trips offered by Carpe Diem are inclusive of air fares, accommodation, transfers and most meals. The price of an eight-day trip is on an all-inclusive basis. To contact Carpe Diem Expeditions Ltd, fax 834-6646 or phone 834-8442.