Radak: Because Hashem is with me, I do not fear anything that a man wants to do to me, for he lacks strength and ability.

Malbim: Had Hashem answered him amidst constriction, he would fear lest even though his punishment is lightened, He will not totally save him. However, now that He saved him expansively, he knows that just like his Tefilah about the illness was accepted and a miracle was done, also a miracle will be done against Sancheriv - "Mah Ya'aseh Li Adam."