I live in nearby Dallas, and I have many of the same interests and challenges as you do. I do not have a family, however. Last year I earned about 90K. Accordingly, I am also looking for a career that will both hold my interest and pay well. It is nearly impossible to save for retirement on say 45K. Not that I care to retire, but health issues eventually force one into it.
During the past couple months I checked out a few Texas nursing schools, including UTA, which I visited. Let me be very clear in warning you that admission to nursing schools is becoming more competitive every day. The word is out about the so-called nursing shortage. UTA had over 600 applicants for their upcoming 100 openings. I found the same ratio at UTH when I visited there recently--over 700 applicants for 144 spaces in the Fall of 2003, and the application period is still open. Until the economy fully recovers and the media stops promoting the artificial nursing shortage, the applicant pool is going to increase. You might as well assume that in the near future the applicant pool will swell to 1,000 candidates for each 100 openings.
Regarding the MDA vs. CRNA choice, you have to do what interests you more rather than think in terms of time and lost wages. For example, if having the title "doctor" has always meant something to you, then you might feel as if you settled for second best. Which type of work interests you more? Perhaps you don't know at this point in time. Continue researching both careers. You will be in either one a long time, maybe forever. MICU had a good point about being in charge. As a director you are one who is by nature a leader, whether or not you realize it. Keep this in mind.
You can live off loans, but it can be difficult. For the past 4 years I have recorded my income and most monthly expenses in an Excel spreadsheet. I suggest that you do this also, or use some other application, such as Quicken or MS Money. You really must have an accurate accounting of your cash flow and what can be eliminated from your current budget. It is easy to overlook numerous little expenses when you have a good salary. Example: I love to buy hardcover books. Often I have spent $100 on them month after month. It sure is easy to do when you earn 90K per year. Consider spending an additional year or two at your current job to save money for when you will need it later. Herein is a possible strategy. When the economy recovers and the media moves on to other issues, the nursing school applicant pool will decline. You could time this opportunity by staying at your current job a bit longer. Admission will be less competitive later, and you will have saved more money.
As others have advised, do not worry about your age. Young people have little idea of where they are going. They cannot imagine having several careers throughout their lifetime. I too am in my 40's. Changing careers at this age might not be easy. But you know very well what risk is about. Like you, I too am tired of being in a career where every day you wonder if it is your last. Which is more risky? Changing careers in middle age or becoming unemployable in middle age?