We leave NJ around 6:30 a.m. to head for Lorton (if the train departed Sanford the night before on-time). That gives us time for a breakfast stop, and we generally arrive at the beginning of check-in time for the AutoTrain.
If the weather's nice, we check in, then walk with our overnight pull bags over to the nearby shopping center for some lunch -- good sports bar and good sushi place over there (as well as 1 or 2 more restaurants we haven't tried). It takes us about 20-25 min to walk there, maybe 1/2 mi walk? We have little ones, so it is hard to be accurate. If the weather isn't nice, we drive over, checking in after lunch.
Be sure to check the "latest check-in time" for your type of vehicle, and plan for some traffic just in case. We can always just grab a quick sandwich at the station for a late lunch if we end up in a jam.