Bethesda was a city in Maryland somewhere along the Rockville Pike. Peri Brown stayed in the Bethesda Marriott hotel twice: once for her aunt's wedding and once when she became separated from the Sixth Doctor two days before Christmas in 1981. Bethesda was evidently very close to Washington DC, as she bought a t-shirt emblazoned with that city's name in the hotel's gift shop. (PROSE: Blue Box)
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Bethesda.