Anne and Wentworth were facing a long engagement. They would not have gotten married until Captain Wentworth had earned enough money to support their children (or either of them had conveniently inherited some).

Consequently Anne would not have been left poor with a couple of babies if Frank had died early.

In Chapter 4, Lady Russell thinks:
Anne Elliot, so young; known to so few, to be snatched off by a stranger without alliance or fortune; or rather sunk by him into a state of most wearing, anxious, youth-killing dependance!

Note the or rather. She is not going to be snatched off right away, instead she is going to have to sit around at Kellynch waiting an unknown length of time for this to happen.

She was persuaded, that under every disadvantage of disapprobation at home, and every anxiety attending his profession, all their probable fears, delays and disappointments, she should yet have been a happier woman in maintaining the engagement than she had been in the sacrifice of it; ... Not marriage, engagement.

I am limited in other proofs of this by the restriction to the first four chapters, however I think I can refer to Chapter 23 for some more evidence that they were planning a long engagement rather than a wedding.