CENTRAL OTAGO RAIL TRAIL ON A BIKE FRIDAY
160kms of big country along an old railway route in Central Otago, New Zealand - tunnels and bridges intact. The route attracts 150,000 bikers and walkers each year; about 5000 ride or hike the entire length. The gradient is easy- about 1:50 (suitable for the old steam locomotive) though the surface is unsealed and rough in (a few) places. 
You can get accommodation at the plentiful small townships en route, but it might pay to book ahead in the busy summer/early autumn season early in the year. The area has experienced a small renaissance since the Rail Trail opened. Most cyclists hire tour companies to carry their luggage between destinations; the "real" bikers cart their own. Few trees or shelters en route. Hot in summer; cold in winter. Peak season, March. Winds can be high at times.
Start at Middlemarch or Clyde/Alexandra. The trip is marginally easier from the Middlemarch end. Take the Taeri Gorge Railway (2x a week and a treat to ride) to Middlemarch from Dunedin. You can catch a bus back to Dunedin from Alexandra at journey's end. You can take your time or do it all in one day if you're crazy!
Summit at Wedderburn, then downhill all the way.
It was fascinating en route to find old apple trees growing alongside the rail trail. They had obviously sprung up from the seeds of apple cores thrown out windows by anonymous passengers maybe 60 years before. The apples were sour to eat but the trees receive no care.
You can buy a $10 passport (below) and stamp it at all the old stations (18 of them).
From Alexandra we travelled on to Clyde and returned along the flat-to-undulating Clutha river trail. Well maintained and fun to ride (24km there and back).
