George and Lynne are out for a walk when they come across this large house. It would be safe to assume that this house belongs to Andy from the running club but we do not have all the facts. A logical explanation could be that George and Lynne have been invited to this house for a drinks party, as have Andy and his new girlfriend. Alternatively, Andy and his girlfriend are currently house hunting and have their eye on this large house, whereas George and Lynne are merely gazing in wonder at the large houses in the area.
The fact that Andy is going out with a woman half his age suggests that he has wooed her through his wallet rather than his looks. He looks early 50s which would mean that she is in her mid to late 20s. If she is attracted to his money, and that is very cynical for us to say but quite plausible considering the other pairings that we have seen in the Wimbledon area - it's basically full of floozies and sugar daddies - then it seems that this is his house.
Lynne comments that Andy's girlfriend has twice his stamina. She says this because she looks relatively calm whereas he is lagging behind and struggling to keep up. If we look again we can see that Lynne has her maths all wrong. If he were to have half her stamina he would either be right next to her and out of breath having had to run to keep up with her or lagging behind and out of breath because his walking speed was not as fast as hers. It appears that he is not only lagging behind and out of breath but also running to keep up. This would make him have a quarter of her stamina. What must he be like in the running club when grown men are running?He definitely won't be able to keep up there. It's a wonder George knows who he is. Then again, many people in the running club must ask "Who's that fat man lagging behind?" to which the answer will always be "That's Andy."