Is the altitude really that bad?

Yes. If you fly directly from sea level to 3,400 meters, you will feel it. Prioritize hydration and avoid alcohol for the first 48 hours.

Do I need a tour guide for ruins?

You can explore most sites on your own, but hiring a licensed guide at the entrance of major sites like Saqsaywaman costs about $20 and provides context you'll miss otherwise.

Planning a Perfect Week in Cusco

To see Cusco properly, spend your first three days at lower altitudes in the Sacred Valley to acclimate before heading up to Machu Picchu. End your trip in Cusco city to explore the historic center and local markets once you are fully adjusted to the 3,400-meter elevation.

Prioritize acclimatization. Do not stay in Cusco city on your first night. Take a taxi directly to the Sacred Valley (Ollantaytambo or Urubamba) which sits at a lower, more manageable elevation of roughly 2,800 meters.
Book your train and entry in advance. Secure your Machu Picchu entry tickets and PeruRail/Inca Rail tickets at least 3 months ahead. You cannot buy these on the day of arrival.
Map your transit. Use 'collectivos' (shared vans) for cheap transit between Sacred Valley towns. They cost about 10-15 soles per ride and run constantly from main plazas.

Is the altitude really that bad?: Yes. If you fly directly from sea level to 3,400 meters, you will feel it. Prioritize hydration and avoid alcohol for the first 48 hours.
Do I need a tour guide for ruins?: You can explore most sites on your own, but hiring a licensed guide at the entrance of major sites like Saqsaywaman costs about $20 and provides context you'll miss otherwise.