I have worked probably the same amount of hours when I was younger but due to other responsibilities that came up after I got married, I had to stick with the 35 - 40 hours work weeks.
I was recently involved with a very small shop (5 people, 3 full time developers) and it really took a toll on me. I got caught up in a project that had no end in sight and really caused me great frustration and a sense of not accomplishing anything which contributed immensely in my decision to walk away.
Right now I'm taking a long break but still update myself with what is going on in tech & programming in particular. I have always been a coder (only job I ever had) and I can't see myself doing anything else.
BTW, I just turned 50 myself. So here's to another 20 years of writing software. Cheers!
Having said that, I've also been burnt out working for startups. You can do 50-60 hours a week for many months but at some point you need a break. I've done it for 18 months but after that, I was pretty useless for several months.
One way I take a break is to work as a software contractor (which I'm doing now.) This pays well and is almost always a 40-hour per week job. It also tends to be pretty boring but it gives me a chance to recharge.
BTW, to relate this back to the topic, I just turned 50 and hope to continue writing software for at least another 20 years.