I ran a circuit like this at 200kHz before (using something like MJE18008, or something... see below). That was with nice and crisp base drive from the feedback winding. That might not be available here (the resistors in the feedback circuit seem to suggest so!), and a much lower frequency would seem to pair with the unusually large transformer.
So hey, maybe A_L is even more than that!
Another way to approach it: find A_e for the core in question, and find out what the lowest frequency is before saturation (or some modest value of Bmax like 0.2T). Then solve for air gap, and therefore A_L.
Probably, somewhere between these two constraints will be the actual intended value.
(Ref:
http://seventransistorlabs.com/tmoranwms/Circuits_2010/RegBO.pngTransformer was some oddball EE core pulled from, I think, an ATX PSU's 5V standby circuit. I didn't have to gap it, and it didn't seem to have any gap at all, but the A_L was just right for this purpose -- not sure if it's a low-mu ferrite, or a powdered iron EE shape, or what!
Also, no active current limit. Living dangerously! ...As it happens, I built a PCB for this thing, back in the day; but I never really got it to work, and didn't try to fix it.)
Tim