There are some great places in North Carolina -- maybe just within 8 hours, but maybe closer to 10. Westglow is wonderful -- definitely luxurious. http://www.westglowresortandspa.com/

The Old Edwards Inn and Spa is also lovely and quite luxurious, but slightly less expensive than Westglow.
http://www.oldedwardsinn.com/

I think there are other good options out there. For example, the Grove Park Inn in Asheville? Have never stayed there though.

Yogaville outside Charlottesville may be too no-frills and hippie for what you want and Kripalu in Mass. and Asheville, N.C. are probably farther than 8 hours, though Asheville might be inside the limit. You may want to check out Nemaconlin or the Bedford resorts in Penn.

Berkeley springs?

For my 40th B-day, I'd love to do a solo stay at some sort of yoga/spa type resort. I'm looking to keep this within an 8 hour drive to DC and not too too pricey (luxury not needed!). Ideally, some sort of place I could stay, take yoga classes, go to a gym, get some spa treatments, hike. Healthy meals on site a plus, but might not be realistic. Any recommendations?