Having completed a couple of runs, I’ve found that what was easily Zone 1 pace pre-accident now puts me into Zone 2, most likely because of the rib pain my doctor mentioned.
Totally normal and largely due to a drop in mitchondrial mass. The mitochondria that are created in five weeks of training are reduced by half in one week of rest (Olbrecht, 1997).
So three weeks off is going to have a big impact on your aerobic capacity. The good news is that it comes back quicker the second time.
A good rule of thumb is to spend at least one day in Z1/2 for every day missed. So plan on at least three weeks before you feel like your back to where you were.
As far as rib pain goes, I would follow your doctor’s recommendations.