Most in-car portable GPS receivers have a pedestrian or walking mode. The pedestrian mode optimizes the route for walking, and may also adjust arrival times to match walking, rather than driving speeds. Use your in-car portable GPS for walking as you would for driving. Select your destination by address, or from a points of interest search, and start your route. You will receive text and spoken directions much as you would when driving.
Consult your manual for the selection of pedestrian mode. In the Garmin line, from the main menu touch tools, settings, system, usage mode, pedestrian, OK.
In the TomTom line, select your destination, then select "walking" as your route type.
Car GPS navigators are useful for street navigation, but they do not have maps suitable for off-road walking navigation unless they are special "crossover" models such as the Magellan CrossoverGPS or the Garmin Nuvi 500. If you plan to do extensive off-road hiking, you will be better off with a handheld GPS receiver.
TIP: note that car GPS receivers typically don't have a long battery life (1 - 3 hours), so if you're on a long walk, turn on the GPS when you need direction, then turn it off to conserve battery life.