Weekend driving used to be fun, an adventure, and exercise in mind over map. Used to be until freeways connected cities, signposts became irrelevant if you knew what exit to take and red lights became as numerous as gnats in July. But there is hope. There is rallying. Any two people with a modicum of common sense, a watch, a pencil and an ordinary car, may participate. Rallying is based on a simple premise-if a car maintains an average speed along a certain route, it will arrive at its destination at a precise time. Not that it's all so simple in practice, which is what makes it fun.The driver in a rally concentrates on the road while the navigator reads the instructions and watches the clock. The goal is to reach checkpoints along the course in a specified amount of time-no more, no less. "It's really an exercise of the mind," says Al Marsh of the BRANDED Rally Club which begins another season of TSD (time-speed-distance) rallies next week in conjunction with Montgomery County's recreation department. To introduce people to the concept of rallying, the club will hold a free workshop April 20 at 7.30 p.m. at the Bethes To register or for information call 468-4217.