In his article on Reston (Op-Ed, July 21), Thomas Grubisich described how imaginative community planning, sensible public transport and use of innovative heating and cooling equipment can enhance the effectiveness of a "new town's;" energy consumption. Unfortunately, you'd need many Restons to alter nationwide realities, and many Restons are not in the cards. The fact is that neither demographic trends nor financial resources allow us to pin our hopes for large-scale energy conservation on such incremental land-use patterns.

The reason is clear-cut: Out of 70 million hou ing units in the country, over 50 million will still be with us in the year 2000. And much of the housing added in the interim will necessarily be tied to today's infrastructure of homes and transport network.

The Washington metropolitan area offers a vivid example of how energy conservation needs to be pursued within the here-and-now. A study for the Metropolitan Washington Council of Governments a few years ago found that the region's total energy consumption under the historic pattern of the most energy-conserving land-use scenaria.

This is not a plea for tolerating sprawl. Rather, it is to show that the energy savings ascribed to an ambitious change in lang-use practices turned out to be virtually identical to what might be achieved by a mere 35 percent improvement in the energy efficiency of cars associated with prevailing land-use trends and practices. And we can do a lot better than 35 percent in automotive fuel-efficiency improvements.

Fortunately, vastly more efficient energy use is possible within the prevailing structure of housing and mobility. For example, an existing single-family house consumes, on the average, about 122 million Btus annually for heating. Even at existing prices of heating fuels (around $2.50 per million Btus), substantial cost-effective conservation opportunities could be exploited. If energy were priced, as it should be, at its true replacement value (say, approximately $3.60 per million Btus), homeowners would become much more conscious of energy-saving potentials, with the result that energy use for heating might easily be cut by a third - to 80 million Btus. The conservation possibilities include insulation, double-glazed windows, zoned temperature controls, night thermostat setbacks and numerous other - more novel - practices.

Even if - as seems likely - private cars continue to be the dominant means of passenger transport, there are various indications that we are approaching substantially slower growth in the total number of vehicle miles traveled, and the projected rise in that crucial figure is more than offset by projected improvements in gasoline mileage. One might estimate the average fuel efficiency of the total fleet of automobiles on the road in the year 2000 at 27.5 miles per gallon, the standard mandated for cars to be produced in the 1985 model year. (The 1976 fleet average was about 14 miles per gallon.) As a result, the nation would consume 30 percent less automotive fuel in the year 2000 than in recent years.

One can easily contemplate more amtitious mpg targets, coming about from more stringent standards than those already legislated, accompanied by some considerablve down-sizing of vehicles beyond 1985.

Public transport has a supportive role to play in making cities more livable and reducing energy use in transportation. But the future may lie less with the Washington Metro or San Francisco BART models; these systems are incredibly costly and serve the limited ridership that straddles high-density corridors. A more dispersed bus network might yield greater payoff. Another promising and practicable transportation system to supplement the private automobile is jitney service: the use of automobiles or vans that are designed to meet local commuter needs. If any new type of short-haul public transportation is to become nationally important in the next few decades, this may be it.

Visionary thinking about desirable and fundamental changes in the energy-using features of urban life is needed and is welcome. At the same time, such visions should not deflect us from hard thinking about the real progress that is possible even within the framework of existing societal preferences and institutions.