There aren’t a huge number of long back-country routes. Me and some mates did the Moerangi route recently, it’s only 35km but has some testing climbs. It’s remote, and you’ll need to organise a ride/pick-up there or back. There’s a farmer nearby who’s set up with a bike trailer and van who can do this, a quick google should find his details.
Alternatively you could spend a couple of long days at Rotorua without riding the same bit twice. A lot of it is very trail-centre-ish but it’s through stunning scenery and there are some great outback-type trails out towards Blue Lake and Green Lake.
Taupo is an hour from Rotorua and there are miles of trails at Craters of the Moon, plus a pleasant trail from Craters down to the Aratiatia dam and back up along river to the lake.
Ask on vorb.org, it’s way, way quieter than STW but there may be some locals to give more detailed knowledge or offer to take you out and show you some trails.