Unwashed duck canvas is stiff on purpose. The fibers are dense and coated with the mill's sizing, and it takes real wear to break that down.
Don't wash it to speed this up. Washing sets creases where you don't want them and pulls the color unevenly before the fabric has decided where it's going to wear.
Just wear it. Two to three weeks of normal use and the elbows, shoulders and cuffs soften first — exactly the spots that need to move. The body follows.
First wash: cold, alone, hang dry. After that it's yours and you can treat it like anything else.
If it gets soaked, hang it somewhere with airflow and let it dry slowly. Don't put a canvas jacket in a dryer. Ever.