This is primarily an amateur astronomer event and is particularly suitable for beginners and the public. It will very much help to first read an introduction to this event and deep sky objects in particular.
It is not so much a race in time, as in who wins first, but an attempt to see all the objects in the list, in one night. You win if you have seen all 110 objects (modern interpretation is that there are only 109 in the list but for consistency across all events the disputed object is included). Many are easy, and the rest tend to increasing difficulty.
Difficulty varies throughout the night. For most of the night, especially during the middle, there is plenty of time to see and most objects are easy. Difficulty is for those that are faint and those close to the southern horizon but the real challenge is to see those objects close to the Sun at sunset and again at dawn; they won't be set against a dark sky.
From Kenley it is possible to see all 110 (109 have been seen throughout a year), but not in one night. The marathon for us therefore defaults to seeing as many as possible (clearly those below the horizon or in too light a sky are not expected to count). From more southern latitudes as in southern Europe, the USA, middle and far eastern countries it is feasible to see them all in one night.
The most challenging will be those very low in the sky, and these few are in the constellations of Scorpio and Sagittarius, visible only in summer. Usually it will be buildings and trees that will intervene. It can be said that all but two have been seen with the main telescope at Kenley, and the last two escaped at a time when the dome was under repair during summer 2014. It will involve seeing them between gaps in the trees to the south.
Various members of the society have seen most on the list in one night. A moonless night is certainly preferred but not essential, if the most difficult are not affected by moonlight (it is sufficient to have seen the object, not in how well).
What makes the marathon work is that there is a gap in the distribution of objects across the sky, and if the position of the Sun occupies that region, as it does during late March and early April, then all the objects can be seen in darkness or sufficiently so.