How many of you guys have played sports at a high schoool or college level? I ask only because - The most endurance related sport I ever did was wrestling. Similar to tennis in terms of explosive movement, and a need for stamina. We warmed up with 3 mile runs. Every day, 6 days a week. That was nothing, just the start. We wrestled and did drills..etc for 2 hours then closed with HITT drills. You think it didnt help? Honestly, it was a must to make weight and then survive on the mat. To me, tennis is similar. If your opponent is as consistent as you, it is a battle of will. Who loses focus first? Who cant handle the summer heat? Thats why you run. Sorry I sound like LeeD here but the OP is a very nice high level view of what needs to be done.