WHEN I WAS a student at Columbia University, I got up early one morning to tag along with Harry Truman, then recently retired from the White House, on one of his celebrated morning walks. Stepping along at a staccato-like 90 steps a minute, a pace he said he'd maintained since his Army days, he chatted amiably, briskly answered questions and tossed out pungent observations about life not meant for a family newspaper.

A few years later in Berlin, I went to the eastern side for a walk and a first-hand look at how the ceremonial guards do their exaggerated goose-step. Recrossing Checkpoint Charlie later, I was puzzled to see a clutch of American tourists applauding and looking at me. Then I heard one of them shout, "Art! I'm from Malibu!" I turned around and saw a grinning Art Linkletter a few steps behind me.

Once while living in Austria, I took a walk in Vienna's Lainzer Tiergarten, a vast park run on the quirky notion that people and wild boars can coexist. After seeing one snarling boar menace a terrified child, I headed for a gate several hundred yards away only to find that a particularly nasty-looking boar was stalking me on a course that would meet mine well before the gate. I doubled my Truman-like pace and by the time I got to the gate, the boar had found someone else to stalk.

I have always loved walking -- for pleasure and for exercise -- and I've tried to do it wherever I happen to be. Not many of the walks turn up a Truman or a Linkletter or a wild boar, but there are always other delights or new experiences or just plain good feelings not available to the motorist, the bicyclist or the jogger hurrying by.

When jogging first became the rage, I briefly tried it. But I soon noticed that almost no joggers smiled while nearly all the walkers I met did.

Backpacking has never tempted me. Since Boy Scout and Army basic training days, I'm never gone long enough to use the paraphernalia the serious backpackers lug along with them, although I sometimes put a sandwich and a banana in a bookbag-size pack if I'm going on a trail in the country.

So I'm a walker, not a jogger or a backpacker. But neither am I a casual stroller. My walking is purposeful, systematic and energetic enough to form my main regular exercise program.

I take a four-mile walk several times a week. I may take longer hikes on weekends, but I've found the four-mile walk ideal for my own regular walking regimen.

For one thing, at my average pace, I walk a mile in 15 minutes. That means four miles takes one hour, a block of time that can be handily scheduled into anyone's normal day -- before work, at lunchtime or in the evening.

Also, my neighborhood seems to lend itself to convenient four-mile routes. From our house, near Glen Echo, I've plotted four -- two hilly and two, on the C&O Canal towpath, level. I also have a circular, two-mile "threatening weather" route that puts me no more than six minutes from home and shelter at any time.

I keep a running total of my mileage -- not because I'm a compulsive record-keeper but because I've found it keeps me from getting lazy. I started two years ago when I realized I was falling off my already-casual walking schedule. Now if I miss a couple of days, the numbers are there to scold me and prod me out of bed the next morning. And now I can have a goal to shoot for -- topping last month, or the same month for last year.

Does this approach to walking rob it of its joy? Not at all. No more, certainly, than a serious pursuit of improving a forehand or perfecting a backstroke or breaking par does.

And regardless of how it's done, walking requires so little concentration that, probably more than any other form of exercise, it can be done while the mind flows free. I have relived old times, laid plans for future times, plotted out speeches and articles (including this one), brainstormed solutions to family problems, mentally hummed long-forgotten tunes and just plain daydreamed -- and found that many of my best ideas have come as I've walked, with my mind just along for the ride.

Here are some suggestions and some things I've noticed about walking:

ROUTES -- The Washington area is blessed with a rich variety of attractive and practical routes, with both hilly and flat terrain within walking distance of most neighborhoods.

While not everyone lives within easy reach of such treats as the C&O Canal or Rock Creek Park or Anacostia Park or Northern Virginia's network of bike trails, there are few places hereabouts that have no interesting streets or other routes a 15-minute walk or less away. Much of the fun of starting a walking regimen is finding places close to home that you hadn't thought about or explored before.

Try this: Take a street map of your area and, using a drawing compass or a pencil and string, draw a circle, centered on your home, with a two-mile radius. You'll be surprised how much it takes in. Within that circle, using your car's odometer or a pedometer, you can chart some four-mile trips that begin and end at your home -- the most practical kind for daily exercise. One-way four-mile routes from your house can put an even-more astonishing number of places within reach of your own feet -- and you can arrange to have someone retrieve you or use public transportation to get home.

Some favorite routes of mine are Rock Creek Park, the delightful and little-used Glover-Archbold trail that slices through Northwest, the C&O Canal towpath, and the vigorous and exhilarating Billy Goat Trail, off the C&O Canal just above the Old Angler's Inn.

For almost anyone, solitude on a trail (at least on weekdays) is only a few minutes' walk away. Near the posh Fort Sumner area of Bethesda, for example, there is a section of the single- track B&O freight spur just off the Little Falls hiking path that is secluded enough to be a lonesome track anywhere in the rural Midwest -- although two shopping centers and the homes of the last three secretaries of state and the present speaker of the House are within tooting distance of the line's rare locomotive.

EXERCISE -- With walking, as with all exercise, the key is the amount of exertion. Most experts seem to agree that for walking to benefit the cardiovascular system, it must be brisk and, at least in part, strenuous. At its normal level-terrain cruising speed (about 3 to 3.5 miles an hour for most people), the body doesn't get much of a workout. But by walking up hills, increasing your pace a half-mile or more an hour or carrying extra weight such as a backpack, you can boost the exertion -- and the exercise benefits -- significantly. Swinging or pumping the arms is important to benefiting the upper body, although it will probably embarrass you the first few times, before it becomes a natural part of your stride. (In Autria, I once went to the other extreme, trying to fit my body to the Central European custom of walking with hands clasped behind the back. The next morning I awoke with sore armpits and I was well into a worrisome day before it dawned on me that it wasn't a lymph- gland infection that was hurting me but simply my American muscles unused to being stretched to let my hands meet at a point where European hands seem to meet naturally when left to dangle.)

My own weight, respiration and blood pressure are measurably improved as a result of walking, and there are a number of books that examine the health aspects in detail. One that I found to provide a truly exhaustive treatment of all aspects of walking is "Rockport's Complete Book of Exercise Walking," by Gary D. Yanker, available at Eddie Bauer and similar outlets.

EQUIPMENT -- This is the best part, because none is really needed, although a good pair of shoes is a sound investment. Walking can be done in any kind of clothing and you need a backpack and canteen only if you're out for more than a couple of hours. My own investment in walking equipment tripled recently when, after a flareup of a painful heel spur, my doctor told me to stop wearing my department store sneakers and move up to some fancy jogging shoes whose side supports at the heel have made a world of difference. I have bought and tried a radio-cassette headset but quickly discarded it as a physical, mental and aesthetic intrusion.

ACCOMPANIMENT -- My preference usually is to walk alone, but there are frequent exceptions. My wife and I have found, for example, that we have some of our most productive conversations while puffing up hills together. Somehow, too, an argument can't seem to last, let alone start, when you're hiking a footpath together. My two sons, 7 and 8, can match my distance but not my pace, so periodic rest and catchup stops are needed or, on paths like the C&O Canal, they can ride bikes while I walk.

I have found no way to combine exercise walking with walking a dog, if it is done legally -- and properly -- with a leash. There are too many interruptions and changes of pace, not to mention confrontations with other dogs. Even worse, however, can be the friendly stray that decides to be your pal for a walk and won't take "Go home!" for an answer. One of the low points of my adult life was when such a dog, after ostentatiously walking beside me for some distance, dashed toward a row of brand new houses where a vast crew of workmen were doing the final smoothing of a half-dozen freshly poured concrete driveways. A chorus of curses erupted up and down the block at me and the dog as it waded in, and I knew that even if they could hear me, no one would believe me telling them it wasn't mine. So I threw my shoulders back, pumped my arms, accelerated my stride, outpaced Truman and the wild boar and finally was out of earshot and puffing healthy aerobic puffs, mentally humming a jaunty old ditt and letting my mind flow free.

Richard Homan is an assistant foreign editor of The Washington Post. TOGETHERNESS ON THE TRAILS

If you'd like to take a more "organized" approach to walking and hiking, there are a number of opportunities:

AMERICAN YOUTH HOSTEL outings require advance registration. Call Potomac Area Council, 783-4943, for newsletter.

ARCHEOLOGICAL OUTINGS or nature treks most weekends at Ellanor Lawrence Park in Centreville. Free. Call 631-0013.

BACKPACK OUTING with Wilderness Walks. Booking for Scotland walk Aug. 17-29. Call guide Al Schneider, 301/972-1582.

BACKPACKS FOR WOMEN weekends through July 8 with Great Encounters. Call 546-4654.

BIRDING & CLIMBS most weekends with naturalist at Riverbend Nature Center in Great Falls. Free. Call 759-3211.

CAPITAL CLUB outings most Sundays via chartered bus leaving 1424 K St. NW. Call 522-2764 or 686-6338.

CENTER CLUB has utings most Saturdays, Sundays and holidays. Sometimes short in-town hikes weekday evenings. Call 301/949-2418 or 301/891-2369.

DIAL-A-HIKE, 547-2326, gives information on Sierra Club outings in the Washington area.

MARYLAND MOUNTAIN CLUB Saturday and Sunday outings most weekends. Call 467-7398.

NATURE OUTINGS most weekends at Hidden Pond Nature Center in Springfield with naturalist. Usually free, but reservations required. Call 451-9588.

PENTAGON PACESETTERS -- has schedule of upcoming events. Send SASE to Pentagon Pacesetters, P.O. Box 46089, Pentagon, Washington, D.C. 20050-6089.

POTOMAC APPALACHIAN TRAIL CLUB has tape recording of upcoming outings & worktrips, 638-5306.

POTOMAC BACKPACKERS has trips most weekends. Call 836-2921.

PRINCE WILLIAM FOREST PARK has hikes with rangers, orienteering sessions. Call 703/221-2104.

SENIORS WALKS with Walking Sticks Thursdays, leaving Culpepper Garden Center in Arlington at 10. To monuments, parks, historic sites, scenic areas. Free. Call 524-1144.

SIERRA CLUB Patuxent Group outings monitor Patuxent River cleanup needs. Some other outings, Bowie & Upper Marlboro. Call 301/490-1978.

WANDERBIRDS trek most Sundays via chartered bus leaving 17th & K streets NW. Call 554-9425 or 301/294-0014.

WASHINGTON WOMEN OUTDOORS have outings most weekends. Call 797-8222.

WILDLIFE OUTINGS most weekends with naturalist at Huntley Meadows Park in Hybla Valley. Usually free, but reservations required. Call 768-2525.

WILDFLOWER TREKS most weekends with naturalist at noon at Colvin Run Mill Park in Great Falls. Free. Call 759-2771.

WILDFLOWER WALKS or climbs most weekends in Potomac Overlook Park. Free. Call 528-5406.