Like a previous commenter said, Air Canada is telling you that you fit their minimum connection standards. Under normal circumstances, 70 minutes will be plenty of time to get from flight to flight. But if that inbound flight is delayed, you're cutting into what's already a fairly minimal connection time. And it's not at all unusual to have significant delays out of Washington. It's nice to have that one-flight buffer, but if that flight is fully booked, you'll be bounced to a later one—you won't be prioritized ahead of passengers with confirmed seats.
Overall, while you're probably going to be okay, it would make sense to investigate your options, even if just for peace of mind. Then you'll have to be willing to put a price on that peace of mind, since flight changes aren't cheap.