Lee and Johnston both had similar strategies, both for the 1864 campaign in particular and the war in general. Both recognized that, given the enormous disparity of resources between North and South, the only way for the South to win the war (barring foreign intervention) was by holding out long enough and demoralizing the Northern population to such a degree that they forced their political leadership to give up the fight. 1864 was the ideal time to achieve this, given the presidential election in the North.
But both had different approaches. Lee's approach was to attempt to win dramatic and decisive battlefield victories over the principal enemy field army - on occasion he seems to have contemplated the possibility of destroying the enemy army in a Cannae-like victory and capturing Washington City itself. His thinking was that this would so dishearten the Northerners that they would agree to peace. His attempts to achieve this not only ultimately failed, but cost his army extreme casualties that the South could not afford.
Johnston, on the other hand, focused on preserving his manpower and resources of his army, so as to keep his force a potent one for as long as possible. So long as the Confederate armies remained undefeated and the prospects of the war ended anytime soon appeared remote, eventually the North would give up. Where Lee felt the need to take risks, Johnston felt the need to remain cautious, to strike the enemy only when success appeared likely enough to warrant possible damage to his forces.
Johnston's strategy, like Lee's, was not successful. What's different is that, due to the nature of his removal from command, the defenders of Johnston have always been able to claim that his strategy would have worked if only he had been given a fair chance to achieve it. Hood, for his part, attempted a version of the Lee Strategy in Georgia and Tennessee, which resulted in the destruction of his army. Since Lee never was removed from command, he doesn't get the "get out of jail free" card that Johnston was given by history.
I've given this question an enormous amount of thought over the years, given that the idea of Johnston remaining in command if the plot of my novel Shattered Nation. But it's a very interesting question that has not received the attention it deserves. Was there a "Lee Strategy" and a "Johnston Strategy" and perhaps also a "Longstreet Strategy" and "Beauregard Strategy"? I think there were and that historians need to more closely examine them.