Hopefully it has finished converting by now

but here's an explanation.
Those percentage indicators? they don't really mean much, except that certain checkpoints have been reached. Conversion only updates it about twice, so it goes from 33 to 67 to complete, I believe as it passes through the Input Plugin -> reparse -> Output plugin stages.
The longest conversions on record were about a day long, this usually happens when the markup is very complicated and requires a lot of thinking

your EPUB seems a prime candidate.
Feed the process RAM and it will get done faster -- or at least not crash with an ~OutOfMemoryError. You have a nice chink of RAM, so it WILL (did?) eventually complete I believe -- assuming you didn't give up.
And though I am late

perhaps this will help someone else down the line too.