Actually, it's been the inverse for me ...
My wife and I typically make love for more than 60 minutes, over 15-30 straight with any constant pumping/stroking. When I was younger, this was easier to maintain. I had already developed a great amount of sustainment and could keep a boner for several hours.
I'm starting to show my age and I can't do it for more than a hour now, at least not normally. I take cold showers and typically keep sex off my mind so I stay completely professional (my reputation is everything as a well-known, at least in my field, author and consultant). I can tell my wife wishes I could keep going as long as we used to every time without extra work, but she knows I'm not getting younger. Especially as I'm double my sexual prime, but she's entering hers.
A couple of things I've done ...
1. Hot baths. If I have a hot bath, I find I have natural errections for the next 24 hours, and longer ones at that. I think much of the base of my problem is the taking of "all-the-way-cold" showers. It's how I survive in Florida without breaking a sweat, but it also makes me -- literally -- frigid.
2. Avoid oral and vaginal exposure. Oral and vaginal liquids give me an instant hard-on, and were very nice in our earlier years of making love. I could stay hard 60+ minutes. But now I'm twice the age of my sexual prime, and while saliva or secretions make me very hard, it lasts only about 30 minutes without constant application of stroking/pumping.
3. As much as I say it, I've had to go back to masterbation to keep my member "active." I travel a lot, and am away from my wife 5, 7 even 10 days in a row. While that makes for a good, quality love making session of 1-2 hours the first time, I have trouble getting it up against in 36 hours. My urologist's nurse suggested I start masterbating, to full release, at least every 3 days to keep it more active. It's working, as I can often make love twice, if not 3 times on a day, for over 30 minutes of constant errection.
Now I'm sure many people will read too much into this and think I can't "get it up." I'm talking about "keeping it up" for the very long durations my wife and I make love. Understand it's been virtually never that I just entered my wife, but our love making is lots of re-positioning, and lots of foreplay, including lots of kissing early, during and as we orgasm. I know there are others like me, and being in their 30s, it's not as easy as when they were 18 or early 20s either.
Yeah, if I just put it into my wife and pumped her for 15-20 minutes, that would be no problem, even a few times in a day. But that's now how it works in ProfV's life, and never will. Which is why I have to be on-top of trying to keep myself more like when I was 20.