It may be more worthwhile trying to do better at your current job and thereafter asking your employer for a raise. Or find a similar job with another firm with better pay, try to do well at it, and move from there. May not be very practical to pursue a JD at 40, just for the sake of trying to earn more money. It's hard to predict what the industry will look like in 5 years time, and there is no certainty that you will be able to get a TC and practice law.

Of course, it all boils down to ability at the end of the day. If you are confident of doing well in law school, then it should not be a problem to get a TC and practice law. But then again, if you are confident of doing well in law school, you might as well apply yourself to doing better at your current job, or making a move to another job that doesn't require a 5 year investment.