I wouldn't worry. If the doctor was really worried he would have rushed the MRI.
I first went to my doctor after only 2 days of the headache because I couldn't handle it. She told me to wait until the end of the week and call back. She really thought it was going to go away. When it didn't I called her on a friday and when she got my message she called me back and told me to go to the ER for a CT scan.
Doctors these day wouldn't put anything off if they have any concern at all. There is to much concern for lawsuits. So if the doctor hasn't rushed the MRI order then don't worry. You are now in the hands of your doctor.
And believe me that weekend was hell for me. I got the CT results immmediatly from the hospital and they sent home but not before mentioning some other things that could be wrong. But my doctor was on call that weekend and I think I called more then once because It started to get worst. I believe it got worst from the stress of worrying. So don't worry and relax.