We guys get along very well with the subject of immortality, thank you, because we all share that gene; it's this mortality business that we have trouble coming to grips with.
One thing I did notice, though, was the idea of training. You do have to work up to being able to ride (or hike) a given distance with a given load, and fitness lapses the longer you're inactive or cut back your training regimen. Just because I rode a century this time last year doesn't mean I can do it again if I haven't touched my bike since Easter.
Joe Friel, the fellow who wrote The Cyclists Training Bible, has a similar book for those of us who have eased up to AARP age, Cycling Past 50. This might be a good deal for your DH, provided you can get him to read it and abide by it.
Friel teaches what he calls the 3 Cardinal Rules of Training:
1) Ride Regularly: Ride your bike often; at least three times a week, four times a week gives the greatest improvement in fitness. (You can ride five to seven times a week and will still improve, but not at as great a rate, and you run a risk of burn-out if you're not careful.) At least one of these rides needs to be a "long" ride of at least 90 minutes, extending time and/or distance as you gain fitness and work toward your goal, whether that be a century or a brevet...
2) Ride Moderately: Moderate riding is that level of training to which your body is already adapted, plus about 10 percent. For example, if the longest ride you've done this year is 25 miles, jumping immediately to a 50-mile ride isn't moderate. Another moderate change is steadily progressing from riding flat terrain to rolling hills, to riding longer hills, to riding steep and long hills. Going from riding on the flats to steep, long hills is not moderate. And likewise, you need to get acclimated to dealing with the summer heat; springing off the spin bike in the gym and toeing the line for the Hotter 'n Hell 100 isn't exactly moderate, either.
3) Rest Regularly: Consistent riding also requires frequent resting. This doesn't mean spending all your time off the bike on the couch or in bed, but it does mean planning rest at the right times, such as after challenging rides or hard weeks. Rest taken in adequate doses and at appropriate times produces consistent training and increased fitness. It's while you're resting that your body rebuilds and gets stronger.
The book sets up a good system for becoming and staying a good, steady rider even as we age, and maybe seeing it in that format migh make an impression on the fellow...