The commute to Alexandria would be a toss-up, to say the least. The problem you will be running into is traveling down I-66 as it becomes HOV-only after the I-495 interchange. I do agree that you could consider the VRE options but the cost of the monthly pass is rather high.
Do you, absolutely, have your heart set on Bristow, VA? You may want to consider homes that are closer to the Metro stations or, even, something past the blue line. Woodbridge area offers commuter lots and is about 15 miles away from the blue line. The prices of real estate drop significantly as real estate in Prince William County is much cheaper than it is in Fairfax County (Springfield, Lorton area).
Another option for you to look into is GWRideConnect and while I have not used it personally, I have heard great reviews about the program. http://www.gwrideconnect.org/transit-options/
Let me know if you would need assistance in your home search! I would be more than happy to help!
Julia Borzova REALTOR®, Century 21 AdVenture Redwood
Office: (540) 659-2161
Mobile: (540) 846-9592
Address: 80 Prosperity Lane, Stafford, Virginia 22556
Licensed in the Commonwealth of Virginia