Although the holidays offer a blessed break from routine, their eggnog and other goodies can mean a disastrous detour on Fitness Road. And parties, travel and guests may interrupt even the most ardent exercise routine.

"But you don't have to wait until the new year to regain your holiday-shot shape," claims fitness instructor Betsy Brown, who recently resigned as fitness director for women at the National Capitol YMCA to open Great Bodies! exercise studio.

For those who think "fitness" and "holidays" are a contradiction in terms, she offers these ideas:

* Make exercise a family affair. After a holiday meal, take a nice, brisk winter walk together.

* Don't deny yourself. Take a small taste of a fattening favorite -- then stop.

* Plan an exercise routine that will fit realistically into a busy holiday schedule. Carry on regular running, cycling or swimming if you can. Or put on a good, upbeat 30-minute tape, start with some easy stretches, warm up for five to 10 minutes then let yourself go: Jog in place, clap your hands above your head, do jumping jacks and keep moving. Finish off with some sit-ups and leg lifts. Try this before a big meal to curb your appetite.

* Give yourself the gift of relaxation amid holiday stress. Go off by yourself for half an hour. Switch on calming music, lie on your back with arms on the floor, palms up. If you have back trouble, bend your knees and rest them against each other. Breathe slowly and deeply. Concentrate mentally on each body part -- from head to toes -- and relax.

Brown will offer a free exercise class marathon Saturday, Jan. 8, 10:30 a.m. to 3:30 p.m., to celebrate the debut of Great Bodies! Inc. at 1500 Massachusetts Ave. NW. Classes last 50 minutes. For more information: 364-0955.

Hi-Tech Gift Gizmos: Looking for a last-minute gift for the fitness fan who has-it-all? The December issue of Glamour magazine highlights these fitness gadgets, available at well-stocked sporting goods stores:

The Electronic Pulse Taker, measures your pulse while you exercise, by DNA Medical Inc., about $70; The Shock Watch with "encapsulated mercury face" to help absorb the elbow-shock of ball hitting racket, by Future Sports, USA, about $30; The Pacer 2000H for cyclists to keep track of everything from smoothness of pace to their heart rate, $160; Cal Count calculates number of calories you burn throughout the day, from San Francisco's Sharper Image, $70.