The true answer is fit enough. A moderate to good level of fitness is required. It is a 19.4km walk which starts with a staged climb to Red Crater. You will be climbing nearly 800m in altitude to 1900m above sea level and as a result you may feel the effects of oxygen deficiency (hard to breathe, slight dizziness). This is not common but needs to be considered. The descent from Red Crater requires some co-ordination and balance due to the volcanic ash and scree underfoot. From this point you will be descending just over 1000m in altitude most of which is a good steady gradient. At all times you should consider the possibility that if you do not feel you can do it, turn back! It is better to return to the start(if you have not already passed the Red Crater) than to try and continue and be caught out in the dark and requiring rescue.