I am 60 but very active and fit. Can't overstate the importance of it. It makes me feel young and I am constantly working on new projects, have lots of energy and general positive outlook. Clients do not believe my age when they see me. They think I am 40. Meanwhile at one point (year 2000) I was total physical and mental wreck after working as a lead architect for a company. Went on my own since then.
What I do - My daily exercise routine is - 2 hour cardio
(hike, cycle, swim if summer) and every other day 4 sets of 20 chin ups or 4 sets of 20 triceps dips for strength. Cardio takes time, strength does not as I have bars right close to my computer in my office.
Sure, one day it will all go downhill and I'll croak but for now I enjoy the life just as well as when I was 20.
I feel that from the biological point of view, the brain evolved to serve the body (mostly for locomotion) and it's level of function is tightly related to fitness in general
After quitting full time job in 2000 I started with 1 hr but very low intensity. After 3 month I've lost all of my pounds and was able to go for however long I want. Like 200km bike ride.
I did 2 - 2.5 hours of exercise for a brief time (around a year), years ago. You can't do it if you have children, but otherwise it can be done. I started work at 7:00 and left at 15:00. Adding transportation I'd normally be home around 18:15 in the evening. I think the key is to not have other obligations (at least not to many) and a minimum of transportation time.
My daughter is adult and being on my own since 2000 relieved me from wasting time on commute except occasional visit to client. The only obligation I have is to my wife and often we ride, hike etc together or with friends.
That isn't the point the comenters above are making. They are saying that without good physical health it is extremely difficult, maybe impossible, to decide what your lifr is about and get doing about doing it for most people. Health fails eventually, yes-but being active and health conscious makes the end more bearable. My gym has a dozen or so 80+ year olds that could train circles around your average 20 year old.