AMN,
Thought not as young as you (I'm 49) and probably not as active, I've experienced some of your issue with just plain running out of gas. I'm 6 weeks out of surgery, (2.1 CM removed on March 9) and even though it was done endoscopically so I have had virtually no side effects, I do find I can't carry on as before. I'm not your level of athlete, but I'm active, used a treadmill and ride bikes often, and can (or could) work all day every day but more like chugging along as opposed to being a runner. Working either at the job or in the woods around the house it was nothing to go 12-16 hours or more without blinking. I also spent my entire life being bullet proof, so this has been a learning experience for sure. I've learned I need to pay attention to what my body is telling me. If I start to notice my balance getting off, or I get a headache or neckache, I need to stop what I'm doing for the day. It's been a strange adjustment for one who never had to think like that before. I do share David's problem with neck pain. I was pron eto this before surgery, but now I find it's a real focus area to avoid. I need to watch my posture, and monitor how long I do paperwork or computer work.
My course of action so far is to do as much as I can up to the point it hurts and quit. The point where I get pooped takes longer to reach every week, so I'm encouraged. I'm already back to the treadmill and bike although gently. I can split wood again but not for hours on end. Other chores seem to be no big deal as long as I don't overdo it. I've had many people tell me the time frame to be 100% back to normal can be quite lengthy after a major surgery like an AN. I keep chipping away at my weakness until it is no longer a factor. Best of luck, and don't get frustrated. This was quite a shock to the body, you may be a while seeing a full recovery.
Zach