Maybe I wasn't clear enough. Traveling from North to South is realistic, your idea of staying 2 days max in every city is not realistic.
Following the itinerary of our
South American Dreams trip is a great idea but may not be as straightforward as you think. The trip is fully organized, we travel with our own vehicle, all hotels are pre-booked. If you do this on your own you will end up changing your plans, believe me, some places are very beautiful and have a lot to offer.
Do not approach backpacking with a sense that you HAVE TO do this and HAVE TO do that. Write down some highlights you would really like to see and allow time to go with the flow (more often than not those are the moments you will cherish for a long time).