As I get older, I find riding my mountain bike for longer than three or four hours at a time is just not as enjoyable as it once was.

I know, I know. All you hard-core off-road bikers will scoff at that.

But I have no problem sitting in the saddle for six hours or longer to ride the entire 26.5-mile long McKenzie River National Scenic Trail.

Maybe it's the clear, rushing river water, maybe it's the spectacular waterfalls, or maybe it's the towering old-growth trees.

But most likely, the reason mountain bikers drool over the McKenzie River Trail in the Willamette National Forest is simply the nature of the trail itself.

It starts out easy around the west side of Clear Lake and becomes extremely technical over the rugged lava rock near Sahalie and Koosah Falls all the way to Trail Bridge Reservoir.

Then, just when you're totally cooked and can't go on, the trail turns into a smooth, easy ride along flowy singletrack all the way down to the McKenzie Bridge Ranger Station.

Never too easy, but never too hard, the McKenzie River Trail is a challenging test of endurance that somehow manages to stay fun for the entire six hours, or however long it might take.

On Wednesday, I escaped thunder clouds in Bend to find sunny skies on the western slope of the Cascade Mountains. I started from the top of the trail, near Fish Lake. (A friend from Eugene coming to Bend to visit would pick me up at the bottom of the trail.)

The trail around the west side of Clear Lake is relatively easy, and offers views of the clear, blue lake that attracts hordes of campers and hikers.

After crossing state Highway 126, the trail winds past Sahalie and Koosah falls along a sheer cliff. This area is often crowded with hikers, so mountain bikers should ride with caution.

Several sections of lava rock were just too demanding, and I was forced to dismount my bike and walk over them. But with a full-suspension mountain bike, many of these areas are rideable.

South of Carmen Reservoir, the McKenzie River suddenly disappears underground for about six miles. I felt a little lonely without the sound of the rushing river nearby.

But the McKenzie springs from underground into a unique body of water called the Blue Pool. The bright, blue water is otherworldly. I watched as college kids jumped from the high surrounding cliffs into the Blue Pool below.

From the Blue Pool, the McKenzie River continues flowing south, and the trail follows it. I climbed high above Trail Bridge Reservoir, then surged down a fast descent. The trail then took me hundreds of feet above the river again, past several cliff-side corners where I had to ride slow with extra caution.

Deep in the greenery of the Willamette National Forest, the river provided cool air as I continued along the trail, which cuts through Belknap Springs and Paradise campgrounds before ending near the ranger station.

The McKenzie River Trail actually includes a net elevation drop of 1,600 feet, but because of the technical nature of the trail most bikers find it to be strenuous. I took several breaks along the way for food, water and photos. (Bikers should be sure to bring extra food, water and a bicycle repair kit.)

I was quite exhausted by the end — but I will no doubt return next summer. Or better yet this fall, when the autumn colors will give the McKenzie River Trail even more majestic scenery.