I've been thinking along these lines for several years...
ever since the discovery of potential ice deposits on the moon. Eventually, the lander and transfer vehicle might be refueled from propellants made on the moon.
It wouldn't take all that long to develop and prove the technology sufficient for going to Mars as well. There are issues like the duration of the flight, thermal issues, etc., but no show stoppers. By the time we were ready to send the first people to Mars, the transfer vehicle technology would be well proven. The Mars lander would be a big challenge to develop, but some of the critical systems could be derived (or identical) to those used on a lunar lander. Gary pointed out a good approach for testing possible aerocapture and reentry systems last week.