I did a similar trip in 8 days! LOL...But, I did not go to Isla del Sol (or Lake Titicaca on the Bolivian side)..I went to Puno and went to the Uros Islands and Taquile. I posted a trip report on this forum if you want to check it out. I took the train to Aguas Calientes....if you like to hike yet want to maximize your time...you could hike/climb from Aguas Calientes to MP...once in Machu Picchu, you could climb both Wayna Picchu and Machu Picchu..all that said.....now that I've been there and did the train thing...I'd LOVE to go back and hike the Inca Trail......

If you have more questions, ask away!

You can read my trip report for Bolivia by clicking my name.

The transfer from Puno to La Paz can be done with Transturin to include a visit to Isla del Sol, expensive but efficient.

I actually flew La Paz to Uyuni and got on a 3 day/2 night Salar tour that same day. Allow at least a full day and night to return to La Paz from Uyuni,

I think you can do a week in Bolivia and fly back from La Paz.

Three weeks is more than enough. There are a lot of tourists going from Puno to Isla de sol by bus. In Lima, you have to go to Astrid y Gaston in miraflores- it was listed in New York Times in the top 50 restaurants in the world. Cuzco has a restaurant named Pappa something. The food in Peru is the best in South America.
A stopover in Copacabana near Isla is recommended. La Paz is worth two to three days, for trips to Tiwanaku. La Paz is pretty, despite being in the poorest country in SA.
We didn't make it to Salar. We visited Sucre, which reminded me of Merida in Mexico.
Our flight out to brazil was from Santa Cruz, which one can skip.