For the context of this question, two people are trying to get from point A to point B (and back) where the only viable means of transportation is bicycle (and a tandem bicycle / electric bicycles are not available). The distance between the two points is 20km (40km round trip). Preparation time is limited, we could go visit point B next week, but it is unlikely we'll ever find ourselves in this part of the world again. The bicycles we do have available would be rented, mid-ranged commuter style bicycles.
We'd both like to make this trip. One of us is a daily commuter (15km each way) who is used to longer rides of 40-50km on the weekends, the other rides rarely and has never gone more than 3km at a time. Even at a slow pace, 40km would be far more than this rider could do comfortably.
What tips/techniques (if any) are there that would allow the stronger rider to complete the trip as comfortably as possible?