Let's look at it the other way around.
If it really was Terri at the gym ... that gives her 90 minutes, with the baby in the truck, to get this done.
Maybe we should look for Kyron closer to the house in a pre-dugged grave.
I'm just sayin'. That's tight. And awkward. And really stressful.
And you'd need to work all of that stress off at the gym. And have a shower too.
****************************
Alternatively, she passed Kyron off to others who disappeared Kyron for her - meeting them was the reason for the driving - and then she worked off the stress of all that at the gym.
****************************
Okay, let's say Dede's being "Terri" at the gym with the baby in the gym daycare, so the real Terri can have tryst. The plan is that Dede's already in Terri's neighborhood working, so she'll meet Terri and use Terri's truck (carseat's in it) and take the baby to the gym while Terri has adult time. At least, this is what Dede thinks. She won't speak of it back at work either because she's Terri's BFF. Dede brings the baby back as scheduled - or Terri meets her at the gym as scheduled; they change cars.
Compliments of Dede & Dede's vehicle, Terri gets another 160 minutes to take care of the business at hand.
****************************
Terri either has 90 minutes, or with help, 90 minutes plus 160 minutes to dispose of Kyron.
*****************************
Or ... Terri hands Kyron off and she really has no idea what happened to Kyron after that.