Its tough. Depends on the employer, there are healthcare systems that have standard contracts for all physicians, and gave me a take it or leave it offer-something like 25 miles for 18 months after I leave (essentially ruling out any chance of practicing in the same city). They did not want to get into the business of dealing with their other physicians if they gave me a non compete that was less restrictive than the others.
Some are more negotiable-I was able to reduce one non-compete from 15 miles down to 5 miles. All you can do is ask and see what happens.
I have no experience with this aspect of things....but talking to some more senior people, I got the impression that non-competes are tough to enforce in reality, but you may need to spend lawyer related fees to get it done.
Also, there is no guarantee that things would not change down the line....if you get a good non compete deal now, there is no guarantee that your employer wont ask for a more stringent covenant in 2-3 years from now when your contract expires and you have less leverage.