Do you still need crutches, ftm420? I had walking sticks immediately post op (that's what the hospital sent me home with) and by 3 weeks was managing very short stints without them. I would have thought that the crutches would make walking more difficult, but obviously I don't know.
I'd go with the advice of the others, do your walking in short bursts and build up gradually. Split that 40 mins into 10 minute stints, say, and rest in between.
Keep doing any strengthening exercises you may have been given, but remember that any muscle damage will repair in its own good time, you can't speed up the process.