Distances at sea are deceptive. First off, ships, even lumbering merchantmen (perhaps especially lumbering merchantmen) need quite a bit of room to maneuver--stopping and turning distances are measured in hundreds, even thousands of yards. "Fog Distance" closed convoys up to 2000 yards (apx 1 nautical mile) fore-and-aft, and 1000 yards P&S. This is "danger close" to use the artillery parlance.
At 8kts, a ship covers 266 yards per minute. if the ship in front of you loses power or steering and you put your own helm over in fog conditions, you are 1/4 way to the next convoy file in just 60 seconds. That gives you about 60 more seconds to reverse your helm and try to get back on a parallel course. And, that, only if the ship ahead has not fallen out of line in the same direction as you steered to avoid her.
So, yes, a 20 ship convoy at 2 mile separation is a big block, about 10 x 10 miles. But, escorts with 14-15kt cruise speeds can easily circle an 8-9kt convoy. That, and escorts do not merely march alongside a convoy like a sergeant minding a company. Instead, they are stood out away from the convoy where their sensors work best, where their weapons have the greatest fields of fire.