What to expect
The principle is simple: every day does not need a major outing. After Lima come two nights in Paracas, a short Huacachina stop and a travel block to Arequipa. Later you stay in the Sacred Valley before spending longer in Cusco. An afternoon around the hotel can be a perfectly good holiday day.
How it fits into a journey
Suggested plan: days 1–3 Lima (3 nights); days 4–5 Paracas (2); day 6 Huacachina (1); day 7 return by road to Lima (1); day 8 fly to Arequipa, days 9–10 city (3); day 11 fly to Cusco and take prearranged transport to Urubamba, days 12–13 valley (3); days 14–15 Ollantaytambo (2); day 16 Aguas Calientes (1); day 17 Machu Picchu and return to Cusco, days 18–20 Cusco (4); day 21 departure. Total: 20 nights. Move the final night to Lima if needed for the international flight.