I've gone from filling up my car every 2 weeks and cycling to work 2X a week for part of the year, to driving for my job and having to work hard to get just a weekly fill up. My car gets very average mileage (18 mpg), it's almost 9 years old, and has AWD, which is necessary where I live. I get reimbursed a decent amount for my mileage, which comes to about 200-300 dollars a month.
I try to do minor errands (drugstore, fish market, bakery) by bike for about 8 months out of the year, but major stuff, like grocery shopping, no.
My dilemma is that while frankly, I can afford to absorb the increased gas prices, it is a huge change from my former lifestyle, where first I lived 6.7 miles from work and then about 13.8 miles. While my clients are all around 5-15 miles from my house, my office is about 26 miles, and I have 2 clients near there. I only go to the office 2-3 days a week and I rarely drive my car on the weekends, as we use DH's. My intention is to stay at my job until I am licensed (at least until the end of June 2013, probably longer), but I recently had a talk with the physician at my office, who also works at the clinic 5 miles from my house. He said he could help me get a job there. The only way I could justify leaving my clients would be the "gas prices" argument. And, I would probably have to work a little more in terms of hours. It's a problem for me at this point.