To enjoy your Gillespie/Rabbit Pass trek you need to be very fit, have good balance and not be afraid of heights. There is significant exposure to falling with small footholds and ledges. Sometimes the footing is unstable and hikers need to be good on their feet and be comfortable on rough or slippery ground (you will not encounter deep mud). Hikers should be able to maintain a slow but steady pace for long periods of time. You will only carry your own gear plus sleeping bag (no tent or food and no sleeping bag after day 3). There is plenty of time to rest, swim, and take photos.
Track times and distances in New Zealand can be deceiving. The tracks here are quite slow progress compared to other countries. The nature of the terrain means that you seldom get long stretches of fast travel. Tracks twist through the forest, there are streams and waterfalls to negotiate around as well as steep ascents and descents. In the alpine there can be loose scree or exposed drops. Do not be deceived into thinking that days with short mileage will be easy, days that we travel less that 10km can be more tiring that days we travel considerably more distance. Call it what you want but it is never dull.