A landing isn't inevitable because a glider has entered still air. That's the time to fly at best L/D speed (unless one is intending landing) to where the next bit of lift might be. If one flies into still air (and there's very little of that - it's usually going up or down, and there is often wind) at 4,500' a 40:1 glider can go a long way before the pilot has to even think about picking a field, let alone planning a circuit. And not only does the sink or lift in non-still air alter the best speed to fly, so does the headwind/tailwind component.
But the impact of changes in speed on distance flown can only be accurately answered once the glider type is known, as then it's polar can be used to answer the question. The McCreedy ring mentioned by 'backpacker' has to be calibrated to the glider type - it's no use putting one from a instructional 2-seater into a 40:1 single-seater.