We had a cold spell recently in germany. Forecast looks stable (not erratic like it is often in the East Coast in the US). And the thing is 17C (only 62F) is currently predicted to be the max with cloudy conditions and a night temp of 10degrees (50F). I'd say this is pretty ideal conditions for marathoning. Sure it could be a tiny bit fresher but its not predicted to be humid so I cannot see the weather having much impact at all.