I use Pandoc to convert Markdown to AsciiDoc like this:

pandoc -s --columns=80 --atx-headers -t asciidoc -o out.adoc in.md 

Or, in long form:

pandoc --standalone --columns=80 --atx-headers --to=asciidoc --output=out.adoc in.md 

There is no need to specify the input format because it defaults to Markdown, more specifically to Pandoc's Markdown flavor.

-s or --standalone tells pandoc to produce a standalone file as output.

I set the columns to 80 explicitly, even though the pandoc man page claims that that's the default. But if I don't do it, the paragraphs are in fact wrapped after column 72. I'm sure there is a good explanation for this, but I do not know it.

--atx-headers uses = signs to mark headers instead of underlining them.