I am not basing my opinion that she is out of shape on being overweight.
I weight 95lbs and could transport 85 lbs if I had to, so I already said it was not impossible. But I do not see this specific woman going through that labor and effort (and being totally unseen doing so).
Getting her out of the house and into the vehicle and then out of the vehicle would be the most difficult part. Lifting an dead weight out of the trunk is very difficult.
Of course I may be wrong, we don't know, but I have a hard time seeing her do it and do it undetected, especially by the man that lives in the house with her.
That is my only point is the scenario of her doing all of this work, body, bed, disposal, burial, dump trips etc.... by herself AND Adam not knowing something was going on in his house is impossible.