We have done this and LOVED it!! We ALWAYS stay at the Embassy Suites in Alexandria VA. It is a GREAT hotel and it includes a very nice breakfast in the morning. It has a parking garage with secruity (so you don't have to worry about unpacking your entire vehicle). It is also right across the street (directly) from the metro station, very easy. You just buy a day pass and you can ride the metro all day and you don't have to drive and worry about your vehicle. It is very hard to find parking in the DC area sometimes. The Embassy Suites is very easy access off the highway and very easy right back on. It is about 3 exits to Lorton.
Old Alexandria is very nice too. We walk to a place called Hard-Times Cafe and they have GREAT food (chili, burgers, frys...great food-and not a lot of $)
If you need any more info, please let me know. We do this a lot and this has worked out for us the best.
Also, Lorton Va, is not the best area (it's not the worst-just not the best) I am not sure I would want to leave my vehicle, with all my vacation stuff. Just my own opinion.