Adrian,
I used to play in a 24-hour event, a cancer fundraiser, at a very hilly course in Sydney (Northbridge GC).
The only sleep was a couple of hours'kip between rounds three and four.
All rounds were walked, so about 25-30km of walking, with a lot of it up and down steep hills.
It took a lot out of you -- my main recommendation is to take a good amount of talcum powder and several different pairs of shorts (I find they cause you to rub in different areas, so are better than multiple rounds in one pair) and a couple of pairs of golf shoes.
I think six rounds in a day is doable with some preparation and if not carrying bad knees or a bad back.
Perhaps if your pro made it two rounds of each of your three courses it might be a bit more doable for a greater range of players/ages?