Yes, the biggest misconception re: the IP is the amount of time it takes to come to pressure. Especially with a big chunk of meat like a roast, longer than 20 minutes is definitely normal.
How long will you be gone? The IP automatically switches to "keep warm" when it's done cooking so it should be fine . In fact, with a roast, a natural release is a usually a better choice anyway, and that can take upwards of 20 minutes, so leaving it to finish cooking is probably ideal.