LIE DOWN WITH LIONS. By Ken Follett Morrow. 333 pp. $18.95.

READERS ABOUT to sit down with Ken Follett's Lie Down With Lions would do well to bear in mind the advice that veteran long-distance runners give novices attempting their first marathon -- the opening stages are only a warm-up; the real race doesn't even begin until the 20-mile mark. Follett doesn't take quite that long to get to the real story -- a highly entertaining, fast-paced mix of KGB, CIA, Afghan rebels, Russian forces, loyalty and treachery set in the mountainous wilds of Afghanistan -- but the first third of the book moves at roughly the speed of recalcitrant bottled ketchup.

The heart of the novel revolves around a nicely detailed quartet of characters, each of whom has come to Afghanistan for quite different purposes: Ellis, an American CIA agent, dispatched from Washington to cobble together an alliance of Afghan guerrilla units under the leader Masud to bleed the invading Russians; Anatoly, Ellis' hard-boiled KGB adversary, bent on eliminating Masud, crushing the Afghan resistance and stanching the flow of Russian blood; the Englishwoman Jane, Ellis' one-time lover in Paris, married to a French doctor who is providing medical care to the rebels; Jane's husband, Jean- Pierre, whose medical work masks an invidious purpose.

Getting everybody to Afghanistan takes the better part of eight chapters, which collectively might be better titled "Waiting for Ellis." After completing a successful CIA operation in Paris at the outset of the book, Ellis dawdles in the United States. Meanwhile, Jane and Jean-Pierre set up their medical clinic in a rebel village in the Five Lions Valley region of Afghanistan, where Jean-Pierre cloaks his murderous schemes in humanitarian garb and Jane gives birth (for 11 pages) to their child, Chantal.

It is at about this point, where even the best- conditioned readers may find themselves hitting the novelistic wall, that Ellis, mercifully, appears -- and the real story cranks up.

It's a good one, replete with graphic accounts of guerrilla operations against the Russians, grisly Russian reprisals and the clash of irreconcilable wants: Jean-Pierre wants Ellis dead (or some reasonably grim approximation thereof); Anatoly wants Ellis alive, to determine if he has cemented a deal that could energize the resistance and open a pipeline of U.S. arms and material; Jane, having discovered that her husband is a heel posing as a healer, wants a new beginning for herself and little Chantal; Ellis wants out of Afghanistan -- fast -- before the Russian noose closes, but all reasonable routes of escape are shut, and the only way out is up -- into the nearly impassable frozen mountains.

THE STORY is skillfully crafted, but it's Follett's characters who make this novel click. Ellis and Anatoly are a pair of seasoned pros who swing from opposite sides of the CIA-KGB plate but never take their eye off the geopolitical hardball. Jane, in contrast, is an ideological innocent who, her murderous surroundings notwithstanding, wants to believe in the innate goodness of mankind (even Jean-Pierre); it's her undoing, but it's also her appeal. Jean-Pierre? Jean-Pierre is a swine, but a readable swine.

As Ellis, Jane and Chantal slip and slide across the ice-covered mountains with Anatoly, Jean-Pierre and Russian tracker teams in pursuit, Follett deftly leads them and the reader down a number of skillfully created blind alleys, culminating in a harrowing page-turner of a finish that rivals his Eye of the Needle for sheer suspense.

There are a few lapses. It's not entirely clear why Jean-Pierre has elected to hoof from Paris to the wilds of Afghanistan to even the score for a perceived injustice wrought by the French government upon his father years ago, or why he sees some rtributive nexus between his scheme and that injustice. As a leftist political thinker, Jean-Pierre gets decidedly low Marx. Jane's discovery of Jean- Pierre's hidden Afghan agenda is of the needle-in- the-haystack variety, and the renewal of her affair with Ellis is four infernos the far side of torrid, written all too graphically in a style which reads like the plot outline for "Debbie Does Kabul."

Lapses and slowish start notwithstanding, Follett has woven a highly readable story, rich in detail and full of surprises, an engaging mix of intrigue tinged with the unexpected.