This all sounds depressingly familiar. I'm looking to FIRE at the end of 2013 at age 47.
To address the burn out issue I changed jobs. A change of scenery and a change of pace has helped me get through the last 18 months in good mental health. The challenge of a new job and dealing with a lot of new people has helped. If changing jobs is not practical, consider whether there is room to grow or develop your existing job.
I have been working on my [-]bucket list [/-] list of things to keep me physically and mentally occupied for a while and have been ticking a few things off already - no point in waiting until after FIRE if I can do it now. While some of the things on the list cost a lot of money, there are plenty that can keep me occupied which do not cost much. To give three examples:
1. I joined a team for this year's 100KM Oxfam Trailwalker and spent a lot of time over the summer training with my team mates before completing the course in November last year. As a non-athletic person I got quite a buzz out of doing something like that
2. adopting a cat has also been rewarding (although not cheap - my wife is now getting quotes to re-upholster the sofa which the cat has shredded)
3. I volunteered as an editor for a report done by a charity that provides support to victims of child abuse. (As worthwhile as this is, I found reading the case studies too much and will probably not offer my services again this year)
When my sister was working (and before she had children), she would sign up for non-accredited lectures at local university one evening a week.
Of course, if you really want cheap thrills join a protest march - the bigger and more controversial the better. So long as you don't get charged with something it's great entertainment and free