Starting to go Off-Track

Hi all, my husband and I have done a lot of tramping but always on recognisable tracks to official huts or campsites. We've never just said 'hey, that looks like a beautiful valley, let's walk in there and put a tent up'. But that's exactly what we'd like to do. I've bought a copy of Moir's Guide North (we live in Dunedin so this covers a lot of the area we are interested in). We've got the gear we need (tents, bed mats, cooking equipment etc). What I'm wondering is things like 'how do you cook dinner if it's raining and windy but you don't have any shelter?'. 'How do you figure out how far you can walk in one day without knowing the roughness of the terrain?'. Do you take heaps of extra food so that you can extend your trip if it takes you longer than you thought? I'm confident about interpreting contours etc on maps. Not so good at estimating how far I've come on the actual ground though. Any tips that could help us take the next step into the back country would be much appreciated, thanks in advance. Rhondda.