A little advanced warning that this coming weekend (25th and 26th September 2021) much of the great park will be closed to casual visitors on both bicycles and horses. This is because they are holding two events (10K run on Saturday and Half Marathon on Sunday). Walkers will still have full access but must obviously be wary of where they walk as lots of athletes will be charging about all over the park on both days.
Historical note: Windsor Castle was the start location of the Marathon in 1909 at the summer Olympics that year. The route then headed through Eton, Slough, Langley, Uxbridge, Ruislip, Harrow, Sudbury, Wembley and Willesden, to White City Stadium. In order for the finish line to be placed directly in front of the royal box an extra 385 yards was added to the 26 mile distance and this odd length of 26 miles and 385 yards is now the official distance for all Marathons run around the world.