You are smart to spend a couple of days in Lima - there are quite a few museums that will give you a good background for what you will be seeing on the rest of the trip. And there are some amazingly good restaurants in Lima.
The Sacred Valley is worth several days - if you have any flexibility at this point I'd suggest adding a day or two there and perhaps eliminating Lake Titicaca.
And if you have any altitude concerns, bear in mind that Lake Titicaca is 1,000' higher than Cusco!
