2020 The Long Walk

In book one of this series (2020 After the End) a meteor strike triggered a nuclear war and its parent meteorite passed so close to the earth and was so large it changed the earths tilt and precession. Now the year is two and a half times longer than before and the seasons are so long and so hot or cold they make life practically unlivable if you stay in one place.
Civilisation was destroyed.
A group of friends survived the first "ice age" winter in British Columbia in Canada by travelling on motorbikes to Baja, now they have to walk back to BC to escape the extreme heat of the northern summer.