I have been away for weeks at a time in northern Canada, Borneo, Brunei, Norway and the US. The longest was two months on the north-west coast of Scotland. It's a dangerous thing to do; it's incredible the amount of calories you burn. But you get an insight into how people lived for thousands of years.
You try to select an area where you'll have food and locate a water source; normally the water coming off the hills is good enough. You could get diarrhoea, but simply boiling up a pot of water, so long as there's not any chemical run-off or pesticides in it, is OK.
Scottish shorelines have a wealth of food – everything from mussels, cockles, winkles and limpets to wracks, kelps and various plants. Depending on the time of year, you get mackerel, cod and saithe. You can gather up shellfish and keep them in rock pools, like a living larder.
Once you've got food and water sorted, it's shelter. For the first few nights you're using natural features, like a rock and a few branches, then you're building it up. You can create a ringed wall then apply a roof to it. It could be made from driftwood. You seal it with leaf litter, seaweed or large kelp. It's waterproof, though that depends on the pitch you put on it. As the days go by the shelter improves. By the end of the week, you have a waterproof, windproof roof over your head and a bed, made with a couple of logs on top of each other, which raises you off the ground. Then you add a mattress from spruce boughs. It's incredibly comfortable.
In places such as Canada you're always conscious there are things out there that will eat you. Exposure's going to take you a lot quicker than them. A chap I met a few years ago in Norway went off to Alaska, stayed with a woman then moved on out to explore. He's never been heard of since.
On your own it's dangerous. All it takes is a sprained ankle or a fall through the ice and the temperatures will kill you, then the carnivores will mop up. I've been in Canada when the temperature has got down to -20C. Once you lose the ability to grasp or grip, you're in trouble. If you can't grip anything, you can't get into a bag, put on warm clothing or create a fire. If you can't get that fire going, things are going to get worse.
In the jungle, the predators that are going to get you are the insects. If you don't take precautions you're going to be a walking scab. Misplace a step and you could find a cobra wanting to stick its fangs into you; grab a branch to stabilise yourself and you could bring your hand into contact with a venomous spider. It's very draining. When you get heavy rain, you get landslides. You also get trees coming down; they used to be called widow-makers. I watched a friend almost getting impaled by a branch. He was tying his boots and a branch fell from the canopy and landed upright a foot behind him.
When it comes to going to the loo, it's a blessing if you've got something soft. Snow's good; sphagnum moss is the best. You should be cleaning, especially your feet, armpits, the crack between your buttocks and your groin. In the jungle little bites become small ulcers very quickly and there's a lot of bacteria.
At night, when you're with other people, you sit round the fire and tell stories; on your own, you occupy yourself, creating fishing lines and hooks. I've never been alone with a fire. It's the original television set.