It depends on where in DTD you are asking. DTD essentially spans from DLH to bag check for the two parks. So, if you leave DL or DCA, you'll be in DTD in a matter of moments. If you're coming from the DLH or the DTD parking lot, it's a little less than half of a mile to the main gates of the parks.
In the map below, the red arrow above the 'DTD' label is right on top of La Brea Bakery, which is at the start of DTD.