How far from the runway should a small plane start its descent from 5000ft? I don't fly, don't have a clue. But one of the main characters in my book DOES: it's a Cessna 152, just him at 14 stone and a couple of small bags, all conditions are perfect and he's flown from about 140 NM away. Would 5000ft be about the right altitude?

The question is: how far away is the runway (a grass one on an unlicensed airfield) when he starts his descent? It's a direct approach, no pattern.
For a comfortable decent you generally want to come down at 500 Ft/Min on the Vertical speed indicator. The 152 is doing about 95Kts over the ground (assuming no wind). So you are covering about 1.5 miles a minute to round it off. So you need to drop 5000 ft on your approach to the area. At a comfortable rate that will take about 10 minutes. In that time, covering 1.5 miles a minute, you will need to start your decent about 15 nautical miles out.