Neither snow nor rain nor sleet nor dark of night deters hard-core runners from their appointed miles. But--sans running on ice--outdoor exercise addicts can still work out safely in cold weather if they warm up first.

"Decreased muscle flexibility in cold weather is a prime cause of winter running injuries," says Washington podiatrist and runner Paul Taylor. To avoid injury, "warm up gradually before leaving the house. Runners should stretch before and after running the wall push-up and hurdler's stretch are good basics and begin their run by jogging slowly the first five minutes."

Runners who start cold in the cold are particularly susceptible to muscle pulls. One of the most common winter injuries, says Taylor, is a groin muscle pull that can require several weeks' rest.

Aerobics guru Dr. Kenneth Cooper recommends a pre-running stretch (also to help prevent low-back pain) in his new book, The Aerobics Program for Total Well-Being (M. Evans, $15.95).

Lie flat on your back and raise one knee up toward your chest and hold it tight into your chest for a count of five. Straighten that leg and repeat with the other leg. Then pull both knees to the chest, hold tight for a count of five, straighten both legs and relax. Then push your lower back into the floor and hold for a count of five. Be sure to keep your head flat on the floor throughout.

That's Shoe Biz: Ninety-five percent of women opt for fashion rather than fit and wear shoes too small for their feet, claims New York orthopedic surgeon William G. Hamilton in a Glamour magazine interview. Most women, he says, choose fashion over fit.

The magazine article advises wearing shoes "a half to a whole size larger" than usual, even though they will seem large at first. "Eventually your foot will spread out as it should," it says, "and you'll never stand a too-small fit again."

No Sweat: Too busy to exercise? Desk jockeys and other sedentary-types can work out in the car, on the Metro or at the office, with help from two new booklets.

* Do It at Your Desk (Tilden Press, $2.95) by Washington fitness instructor Shelley Liebman stresses exercises to counter office-induced aches, from sore feet to high blood pressure. Throughout the day she suggests: Keep a "Nurf" ball handy for squeezing or rolling underfoot, lift a small weight while talking on the phone, release neck tension by rotating shoulders and take a few minutes to close your eyes and relax.

Liebman will conduct a "Where Do You Hurt?" seminar exploring exercise solutions to office-induced aches and pains from 7 to 9 p.m. Dec. 15 at the Chevy Chase Presbyterian Church. The $15 fee includes a copy of the book and other fitness materials. Call 462-3399.

* Commuter Calisthenics (Fitness Alternative Press, $6.95) by Colorado fitness instructor Bruce Andrews and his scientist brother Keither details 70 isometric (no movement) and isotonic (slow movement) exercises suited to planes, cars, buses and other transportation. Among their suggestions for eliminating "commuter stupor" during the average, 44-minute daily commuting time:

"Safety-belt Sit-ups": Place back firmly against the seat, lock shoulder harness in place, push torso against harness for 10 seconds, relax for 5 seconds and repeat 3 times.