Converting lat/long distance to miles.

[Camden Daily]

Ok.  I had axed all my projection blocks because I'd converted all my
shapefiles to lat/long (epsg:4326) using ogr2ogr, so I figured I
didn't need them.  Should I still add a projection block to the map
and each layer?

My map file has a 'units' set to dd, since that is how I set my
extents (and I need to set them that way, since I'm centering off a
given lat/long point in mapscript).

Could I set the extents in mapscript using dd, and then change the
units to miles for my distance calculations?