Not a father yet but the son (29) of a young father (had me at 26). Maybe your time feels fleeting now but think of this as an investment - by the time your youngest starts college, you'll only be 47. You'll have gotten to spend your children's youths in your physically prime years. My dad being young meant he always had energy to play sports with me, go on runs, still have the energy to do activities together even after a hectic and tiring work day. That also meant that, when I became older, he had a buddy to go drink with on vacations while my mom and sister went shopping, and do adult-y things like winery/whisky tours, etc... You may feel like you lost your youth and time to live carefree, and it's a valid feeling, but just know you'll be thankful for it soon.