All Garmin units have an "indoor" capability that's exactly as you describe. You don't have to use it indoors, it's called that because GPS won't work indoors so it won't even try.
Garmin and Suunto both have modes that sample GPS infrequently to prolong the battery, while still providing a map. Garmin's is called Ultratrack. Suunto's implementation uses the compass and motion sensor to fill in the gaps.