According to Strava, the country with the fastest marathon times is Mexico where the average upload is 3hr 26min 6sec. The other quickest nations are Norway (3hr 32min 22sec), Portugal (3hr 35min 46sec) and Poland (3hr 41min 30sec).
Marathon race finish times differ between apps, however. Run Repeat records the top three marathon nations as Spain (3hr 53min 59sec), Switzerland (3hr 55min 12sec) and Portugal (3hr 59min 31sec). The UK is ranked at 37.