I think you are incorrect on your question. There were two main burns done using the Lunar Module descent engine. One was before reaching the moon, to position them into a free return trajectory, the other was about 2 hours after passing the moon, to allow them to land in the Atlantic Ocean instead of the Indian Ocean, a difference of about 19 hours.
From the Apollo 13 Mission Report the reason they couldn't initiate a immediate turn around using the Service Module propulsion system was because they would need to jettison the Lunar Module. It was determined that the Lunar Module was necessary to serve as a life boat for the astronauts, and a direct abort was not an option. (page 23)
Using the SPS was only an option with the removal of the Lunar Module (page 43)
It was obvious that we were committed to going around the moon
rather than performing a direct abort because the large [vector deviation] could have
been supplied by the SPS only if the LM were jettisoned, but that was out
of the question.
Because the damage to the Service Module was uncertain, all systems relating to the Service Module, including the propulsion system, was determined to be unreliable and only to be used as a last resort. pages 40 and 49
At this time the spacecraft was on a non-free-return trajectory with
a 62 n.m. pericynthion. The crew had an abort pad onboard which required
an SPS burn of 6079 fps at 60+00 to land in the Pacific Ocean at 118 hours.
At 56+48, the Flight Dynamics team was advised to initiate all of the
return-to-earth planning based on going around the moon, and assuming use
of the LM descent engine and LM RCS, and that we would not use the SPS
except as a last ditch effort.
Perform a PC+2 hour abort burn with the Service Module
Propulsion System for a landing at 118+00 in the Pacific Ocean. This option
was rejected unanimously as being too risky since no data was available
as to the structural integrity of the Service Module. This option was
put into the category of "last ditch".
