I had a medial meniscectomy on my left knee September 2011 and suffered similarly after the first surgery as you where normal walking was impossible. They generally give you two months to get back to "normal" but I knew after two months I was far from normal, so I made an appointment to see a second OS. I made sure the second OS was a knee (only) specialist. He advised a second (revision) arthroscopy in March this year.
Things have improved greatly since then but it is a long journey.
My advice is to make sure you exercise - stationary cycling is particularly good (no weight bearing pressure) and it gets the blood flowing. After 20 minutes on the stationary bike the blood is pumping so well around the body that I can walk around virtually normally and forget I had the operation.
Also make sure you see a physiotherapist regularly, they know their stuff - particularly if you are walking irregularly on the operated leg as this will generate muscle imbalance and tightness.
As you say the most difficult position is fully extended (locked out) leg so this is the last piece in the rehabilitation jigsaw - concentrate on everything else first (strengthening quads and VMO etc).
Don't worry other people have been where you are now so there is hope!