Te Kuiti to Whanganui
There are no shops on this stretch of road, so stock up with enough food and water for the day before you leave Te Kuiti.
As soon as you've left Te Kuiti on SH3, you'll start on the first of the day's three substantial hill-climbs, which culminates in a rapid descent back down to your original elevation. There's a short flat stretch, then the road climbs to 8 Mile Junction, where you turn left onto SH4, leave a lot of the traffic behind you and continue climbing for another 35 km, gaining 300 m of elevation in the process. However, apart from two steeper sections, one to the Kopaki Rd turn-off and the other to the Tapuiwahine saddle, the gradients are gentle and you'll be able to maintain a respectable speed. After the saddle there is just one sharp rise and a couple of bumps to spoil what is otherwise a gentle descent all the way down to Taumarunui. The final 10 km is pleasantly even, as the road follows the Ongarue river valley imperceptibly downhill.
At Taumarunui, camp at the well-equipped and friendly camp site 4 km east of the centre, on the bank of the Whanganui river. If you need provisions, buy them before you leave the town centre.
Several operators in and around Taumarunui offer both guided and self-guided canoe or kayak trips down the Whanganui to Pipiriki. Contact the i-site at Taumarunui for initial assistance. Various durations of trip are available, from half a day and upwards, but a minimum of three days terminating at Pipiriki is recommended to fully experience the peace and quiet of the river, with its towering cliffs, winding course and numerous rapids. The rapids are mostly low-grade affairs that are exhilarating but not seriously dangerous: you might fall in and get wet, but you'd be very unlucky to hurt more than your pride. Wild camping is not allowed - you must stay at DOC camp sites, of which there are many, spread regularly along the river. A few of them offer bunk-room huts in addition. You must book and pay in advance, either by internet or via your tour operator.
The main advantage of a guided tour is the guide's knowledge of local stories, wild-life, and jokes - and the fact that they might do the cooking for you. The disadvantage is the added cost. Self-guiders will be equipped with a set of maps that show camp-site locations and points of interest along the route. The 2-hour return walk to the "Bridge to Nowhere" is especially recommended. If you're travelling by yourself, you must reckon that you won't be allowed on the river alone. This means either joining a guided tour or joining up with one or more other self-guiders; the tour operators should be able to help you in this respect.
Because tour operators have to drive their vehicles empty down to Pipiriki in order to recover their canoes/kayaks and take them back to Taumarunui, you will most probably be able to arrange with them to store your bike and any superfluous gear while you're paddling, and then bring them to Pipiriki for you when they collect your canoe/kayak. The alternative is to get a lift with them back to Taumarunui and continue cycling from there: either to Pipiriki by road (SH4 to Raetihi, then side-road to Pipiriki, 85 km in total); or to some other destination. Cycling from Taumarunui to Pipiriki is also the solution if you don't have the time or inclination to paddle down the river, but you'll miss out on a great experience.
There are no shops along the Whanganui, nor at Pipiriki, so before leaving Taumarunui you'll need to buy enough food to last the duration of your river journey, plus an extra day's rations in case of emergency (e.g. weather too bad for paddling), plus breakfast and lunch for the final stage from Pipiriki to Whanganui. Dense, dark high-energy bread and lightweight, dehydrated meals that require the addition of boiling water are recommended (especially as the water at the DOC sites is advised to be boiled before drinking in any case). Accomodation possibilities are limited once you arrive at Pipiriki, but camping is now possible in the grounds of the old school, where you may also be able to buy a meal and a coffee from the friendly owners. Ring to check the current status before leaving Taumarunui: blissfully, there's no coverage for mobile phones once you're on the river.
The Whanganui River Road follows the course of the river most of the way to the junction with SH4 about 12 km before Whanganui, but the steep sides of the gorge sometimes necessitate strenuous deviations: The road is narrow and undulating, with an unsealed 12 km section including a 200 m climb soon after leaving Pipiriki and another 200 m climb shortly before joining the SH4. But there is virtually no traffic and there are superb views all the way. As you cycle along, you'll pass through tiny hamlets bearing surprising names such as London, Corinth, Jerusalem, etc, that reflect the ambitions of early, competing protestant and catholic missionaries who wanted to record their respective conquests for posterity.
As you approach Whanganui, you need to cross the river to get to the town centre, most of the hostels and the camp sites. This is possible at the first bridge you come to, which looks like (and is) a railway bridge but which has a pedestrian walkway cunningly concealed on the side facing away from you, making it easy to miss. Cross over the river and turn right up Somme Parade to cycle 3 km up-river to the well-equipped campsite idyllically situated on the river bank, or turn left down Somme Parade to get to the town centre and a variety of hostels including the YHA (which doubles as a B & B, should you feel that you need/deserve a little luxury after several nights of DOC-style frugality). The i-site is on Taupo Quay, which is the continuation of Somme Parade in the town centre.