Maarten,
I read your previous posts that indicate that you had a 5cm An removed by suboccipital approach at NYU recently ( I hope I got it right). I also had a huge AN removed by suboccipital approach at NYU 18 years ago and experienced headaches for 10 years afterwards. First of all, suboccipital is notorious for its headaches. In addition, the way the surgeons close it might contribute to the problem as well. I had it partially covered with a metal plate and a muscle flap inserted into the area that does not have a metal plate. This is a very old-fashioned but very well-known cause for those exertional headaches. Please, find out how they did it.
I experienced exactly what you describe and the headaches were unbelievable. I am now preparing for my next surgery as I experienced a re-growth and just stepped up my exercise routine. Guess what, I woke up with an exertional headache yesterday, although a mild one.
Basically, in suboccipital approach, neck muscles are cut and when they heal, they form scars and adhesions that pull on exertion and cause headaches. The way to allow yourself to exercise more is to change the approach and be prepared for inevitable headaches in the morning which eventually will go away once the body will get used to a particular exercise regimen. Once that happens, you will have headaches only when you introduce new exercises.
First of all, do as much stretching as possible. Yoga is really good. Start small, see how your body adjusts. If runnning on a treadmill gives you a headache because of bouncing, walk or fast walk. Be careful with weights, even light ones because they tense the muscles and you need to massage your neck before you go to bed. I have a few of those neck massagers and the shiatsu one works best for me. it literally kneads my neck like dough and breaks up all the tension. In the morning, be prepared with medication - whatever works for you. In my case, Indocin suppositories because I was also throwing up.
It is possible to get back to an active life after this but sometimes it takes a lot longer than we want to.Start small, be consistent, keep a diary and try to note what type of exercise gives you problem. Work up in small increments. Best wishes!
Eve