I know in vegetables and fruit, Heirloom is considered any plant that has been cultivated, consistently and unchanged (non hybridized) for a period of time. It varies as who you talk to. Some say 50 years, others say 100, and another group says "a steady strain since 1945". Why 1945, I do not know.
It would seem to me, and what I have always heard, is that "Landrace" strains are ones that propagated naturally, with out human interference. My thought is, if you find it on a mountain in Afghanistan, where most people wouldn't be, it probably has been there for a long time and would probably be a Landrace. I know, it's weird, but somehow it make sense to me.