FIREing sure did change my health. I was working in a senior role for a big bank which had lots of long hours, heavy travel and stress. At the time, I thought my health was good for my age (53). I quit my job at 53 (had gotten and delayed a buy-out offer), then took a contracting job for 6 months (kind of a half-way-house) and then fully retired at 54. When I fully retired, I noticed a few things right away.
First, my reaction to the freedom of early retirement was to hike a whole lot. Long hikes or trail runs every day. And I got a dog (a few years since our last one had died) who was also eager to walk. By three months, I had a couple of really solid sports injuries from ramping up too quickly. If I did it over again, I would go slightly less crazy and probably still need to go to the physical therapist. Some problems are good problems to have, if you consider the alternatives. Now a couple of years into FIRE, I have way more variety in my physical activity: lap swim, martial arts, hiking, occasional weights, and heavy chores outside. As a consequence, I have more well-rounded fitness and strength (and more well-rounded sports injuries).
I gradually moved to a much more healthy diet and dropped some weight. Though my overall health was good prior to retirement, my cholesterol was trending towards unhealthy. My healthier diet let me drop it into a very healthy zone without drugs. Having time and planning energy to apply to myself and family made it way easier to take that direction. I now even enjoy cooking partly because of general frugality, but also because of health impact and great meals.
Lastly, in the last 8 or 10 years of work prior to retirement, my sleep patterns got progressively worse with travel and over-work contributing. At the point that I retired, I was probably getting a good nights sleep one out of every four days. It felt as though it took 6 months post retirement to sleep normally many nights and another two years to have sleepless nights be unusual. I'm generally non-moody, but found that I feel more optimistic and content when I sleep well. Also seem to have a better short-term memory that I attribute to great sleep.