It's never too early to start worrying about hypothermia, the debilitating and often fatal condition in which an overdose of cold air or water shuts down the fires in the body's furnace.

And up in the north - central Canada, where snow is already swirling, the trees lost their leaves a month ago and river water registers a frigid 40 degrees, the smart woodsman never stops thinking about it. Which may be why Dave Halsey wrote to John Lenzt last month, seeking advice.

Halsey, a 21-year-old McLean, Va., house painter, is traversing the forests and streams of northern Canada by foot, canoe, snowshoe, dog team and cross-country skis. He left British Columbia in the spring and won't be done until he's notched 4,700 miles to Tadoussac, Quebec. Lentz is Halsey's unofficial adviser on canoe matters, having spent summer vacations canoeing in the vast and treacherous Northwest Terrories. But Halsey was in such a rush to leave that his scheduled canoe lessons on the potomac with Lentz never happened. Lentz is still kicking himself.

Around the middle of last month Halsey wrote to Lentz from the tiny town of Athabasca, Alberta, and asked for some hints on cold - water canoeing. Leintz answered in a letter filled with warnings, the most critical of which he underlines carefully. YOU MUST NOT SWAMP. But Halsey, ever in a hurry, left long before the letter arrived. And he spent the last leg of the 250 mile downstream plunge to his destination. Fort McMurray, huddled in a bow of a government motorboat, his canoe and supplies gone and his strength sapped by hypotherma after three straight day of swamping.

Halsey and his current partner, 23 - year - old Pete Souchuk of Chicago, ran into trouble in fast water only 50 miles from fort McMurray. In the end, Souchuk eudured an all - night barefoot traipse through the wilderness to find help while Halsey curled up on the frozen ground and waited to die.

The worst was Oct. 3. They had swamped twice the previous two days, first just below Middle Rapids, then when they tried to guide their canoe through Boiler Rapids with a rope. Both nights they slept on the ground, huddled together under ponchos, their sleeping bags drenched. They lost two of their three paddles and much food and supplies. "We felt our strength going," Halsey said by the phone from Fort McMurray Hospital last week.

On Oct. 3 they pushed off into cold, rainy headwinds. They hurtled around a bend and suddenly were trapped in unscouted fast water -- Crooked Rapids. They swamped instantly, the canoe somersaulted, their dog leaped and swam for the far shore; their boots were sucked off by the raging river. It took half an hour to fight across the current to shore. "The doctors couldn't believe it," said Souchuk. "Ten minutes in that water is supposed to kill you."

It nearly did. Halsey collapsed in hypothermic shock. Souchuk revived him by rubbing his legs and arms, but once awake Halsey dug his hole and crawled in. "I knew he'd die," said Souchuk. "I told him, I'm walking in to McMurray."

Barefoot and in shock, Souchuk began walking about 6 p. m. He walked all night, his feet wrapped in his poncho, as snow fell in the woods. He walked all the next day, until at 2.30 in the afternoon he heard a boat on the river. he hailed it and was heard by two fisheries biologists on research. They took him in.

Beyond that it was all mechanical. A motorboat was dispatched upstream, Halsey was found alive, the two were transported to Fort McMurray and treated for exposure. Souchuk was bedridden for a week, his feet swollen and torn.

Halsey, who has become something of a Canadian legend for his efforts to cross the frozen plains in the dead of winter, says they'll go on, but are turning the canoe, which was found, over to the fisheries department as an expression of thanks. From here on they will talk, at least until spring.

Halsey is so wedded to the complete 4,700 mile trip that he is heading back sometime this month to walk the final 30 miles along the Athabasca to McMurray. He figures the motorboat ride doesn't count.

And then it's on, through the snow, to LaLoche, Saskatchewan and points east.