HOV = High Occupancy Vehicle. It's designed to encourage car pooling. Depending on which road in the area you are talking about, they are either HOV 2 (meaning you need driver + 1 passenger) or HOV 3 (driver + 2 passengers).
Sterling, Chantilly, and Reston are nowhere near the outermost Metro station (Vienna / Fairfax). To get to that station, all involve stretches on either the same interstate you took in from Manassas or the Dulles Toll Road, which also backs up in the morning. Perhaps the one in Reston is that distance from the Park & Ride lot along the Dulles Toll Road that has express bus service. What is the street address of the hotel that claims to be 2 miles from Metro?
Look at the map on the Hampton Inn website of their hotels in Northern Virginia. The one in Fairfax City (no pool or hot tub) is three miles due west of the Vienna Metro. As you'll see on the main map, all the others are an additional five to eight miles further west / north from that hotel.
You should take a look at the Hampton Inn Convention Center in downtown D.C. Their rates are often under $200 a night, and put you right downtown. They have a pool and hot tub.