MOUNTAIN biking in Sun Valley , Idaho , is best seen from the saddle. But a glimpse from a four-seater airplane of the valley’s mountains, alpine lakes, rivers, canyons, forests and lava flows helped set the stage. The pilot, Tor Andersen, who had his mountain bike and fly- fishing rod neatly stowed in the fuselage of the 51-year-old Cessna 182, was joining me for some autumn mountain biking during the harvest moon.