Just for a ballpark sense of the problem, here's a generated flash-to-subject distance chart (from a small utility I wrote for my own use) for a higher-end Vivitar 285HV flash.

The Guide Number for this flash is rated at 120 at ISO-100 by the manufacturer. All three regular zoom modes are shown, plus a column for use of the external diffuser filter at the Normal zoom setting. The "film" speed is set to ISO-6, per JohnRichard's estimate. The Guide Numbers for each 285HV zoom mode are indicated at the top of the chart.

As you can see, even at the likely optimistic manufacturer's flash rating, the distances for standard large format apertures result in very compromised flash-to-subject distances. Generally (see NORM column) from only 5+ feet (at f/5.6, generally wide-open) down to less than 1 foot (for f/32 or greater, apertures required for any reasonably usable DOF). All distances shown are in feet.