The posts by Gardener and Cattusbabe struck a chord. A friend of mine is FIREd (modest tech IPO payoff) and his wife is still working as a veyr very senior nurse in a big hospital. She has numerous health issues of her own, including bad hips and knees, not helped by a bottle of wine (+) every night and 50 or more excess pounds. But she doesn't do a lot of bending over beds any more - she went down the academic qualifications route which the OP is considering and has been able to work regular office hours and sit at a desk for the past 15 years.
Age 55, retired July 1, 2012; DW is 59 and working for 4 more years. Current portfolio is 1950K split 50 stocks/20 bonds/30 cash. Renting house, no debts.