What could you give a two-hour impromptu lecture on?

Childbirth: what to expect before, during, and after, how to cope with discomfort and pain, possible complications, how to avoid unwanted interventions, aftercare of mom and baby.

I used to do it pretty regularly, so I'm sure it would all come back.

A two-hour lecture would be a waste of time. The audience's attention span would not go much further than about 20 minutes.

How we can end pollution by becoming vegetarians and ending factory farms

Anything Harry Potter related and why mayonaisse is utterly revolting.

Arrest procedures

Relationship psychology

Clean renewable energy.

I cant think of any..

Myself

alcoholism, mental illness, and homelessness. They tie in.