The Overland Track is a full-pack trek of six to seven hours a day on the longest days, carrying 15 to 17 kg / 33 to 37 lb. The terrain is rugged and remote, and it changes constantly — boardwalk, timber steps, rock, exposed tree roots, and mud that can be shin-deep after rain, with sections that are long, rough and steep. There is no road access along the track, so once you set out each morning you finish the day on foot. None of that makes this a technical walk; it makes it a sustained one, and it should not be underestimated. You will need a good level of fitness, good health, and ideally some experience of carrying a loaded pack rather than just of walking.
To prepare, start around three months out: an hour of strenuous exercise three to four times a week - cycling, running or fast walking - plus a bushwalk most weekends carrying a full pack at trek weight, building up to 15 to 17 kg / 33 to 37 lb rather than starting there. Choose hills, and rough tracks over smooth ones; descending under load is what catches most people out, and it is the one thing you cannot fake on the day. The best preparation of all is a multi-day bushwalk with a full pack. Please note that in adverse weather, or on departures outside daylight saving time, this trek may be graded Moderate to Challenging rather than Moderate — shorter days leave less margin if the group is slowed, and shoulder-season conditions are less forgiving.