It's a dog of a long way to Puerto Natales, I did it and regretted I did not do it a bit slower because of the amount of time I spent travelling. I went at New Year and Puerto Natales was not that crowded and I had no difficulty organising last minute accommodation and a day trip to Torres del Paine.
Santiago and Torres del Paine you can easily do in 7-8 days, I added in a few nights in Puerto Varas too. I had been to Puerto Varas before but if you are looking for an alternative that is less intensive then that is a great option too. You can drive up Volcán Osorno, visit Petrohue, drive around Lago Llanquihue and stop of in Frutillar.
I would not recommend you do both though as I did