Exactly.

In fact, when Gandalf catches up with Aragorn/Legolas/Gimli in 'The White Rider' chapter, he explicitly tells them that Sauron has committed a major strategic blunder: attacking too early, as soon as he thought the Ring was in play. If he'd kept some forces back to guard Mt Doom, he'd have been alright. Especially because, as later becomes clear, Mt Doom isn't a normal volcano where you could just lob the Ring in from your low-flying eagle. The Cracks of Doom are in a chamber deep inside the mountain.