You don't say how long an overall ride you want.
Another option that starts closer to Woodbridge is the Fairfax Cross County Trail (
http://www.fairfaxcounty.gov/parks/cct/). I've never ridden it and can't comment on its condition, and you can search for it on Google Maps.
As far as hills on the Mt Vernon/4 Mile Run/W&OD trails, there really aren't any significant climbs. The most I would consider a hill is the last mile climb up to the Zero Mile mark on Mt Vernon. There are other fairly gentle rolling hills, and you can get a steep, but very short (about 50 yards), 25% grade on 4 Mile Run at Glencarlyn Park.
For food, the Mt Vernon trail goes through Alexandria, lots of options. If you are going out the W&OD, you can connect in two ways, via the 4 Mile Run Trail at about milepost 13, Reagan National airport to Shirlington (lots of food options), or the Custis Trail at what would be milepost 18, just after Roosevelt Island at Rosslyn (lots of food options). The Custis has a mile long climb out of Rosslyn and goes by Lyon Village at Sprout Run about a mile and a half later (lots of food options). The Custis meets the W&OD at Milepost 4 (no food stops, but there is a water fountain). Going out the W&OD, you go through Falls Church (about milepost 7, coincidently crossing Rte 7 with food choices), Reston at about milepost 18 (lots of food choices nearby, but not on the trail) where you can also connect to the Cross County Trail, Herndon at milepost 20 (lots of food choices). I could go on out to the end at Purcellville at milepost 44.5, but I think you get the idea.
Also, from the Mt Vernon trail, you can cross the Potomac to Georgetown via the Key Bridge and pick up the C&O canal path that continues all the way to Cumberland, MD (132 miles?) or the Capital Crescent trail to Bethesda.