Absolutely.
52 yo here, averaging about 15 miles/week running + 4-7 miles/week hiking. It takes major dedication just to work that amount in, I have no idea how I'd spin up to do 40 mi/week.
And as far as I'm concerned: The cardio benefit is pretty minuscule compared to what it does for the head.
Then again, it's something that just has to be done. Some form of exercise is necessary to stay healthy and sane. I need to sleep and bathe, I need to do housework and pay bills, I need to do all that other adult stuff . . . and I need to exercise. Any option will take time. Like every other obligatory thing, that means less time reading or surfing the web or playing video games or watching TV or whatever. Oh well. Life's like that.
Slowly, and do a little every day. Or at least, that worked for me. I do 3-4 miles a day (longer on weekends) for 25-30 mi/week total. I've worked up to that from zero over 18-24 months.
Me personally, it takes about half an hour around every run, getting dressed, getting water, stretching before and after, showering and changing clothes. longer runs leave me lethargic for a while. A two hour run leaves me exhausted. A two hour run leaves me worthless for an hour or possibly three.
40 miles in 5 hours is amazing. My bare minimum around running time would be 15 minutes a run, and i can't run 10 6 minute miles 4 times a week. I could maybe do 4 10's@9:00. But if i put in the effort, that would come down.
40 miles at 5 hours/week is just phenomenal. You must have world class biomechanics. I can't imagine how fast you run. You are a stupendous badass. I hope you recover soon.
(As a much slower runner, I can maintain 7:30 for a mile or two. Just not 40. :-))
A very good but not world-level time for the marathon is 2:20 or so for 26 miles. That's more like 5:30 per mile. World record pace is an even more insane 4:41.5 a mile (http://www.runnersworld.com/newswire/dennis-kimettos-maratho...). So yes, 6:30 should be easy, if you're at that level (must be, given that I could do 10k in 40 minutes, which is around that pace, when I was a lot younger, and I'm not a running talent)
The 7:30 pace had my heart rate at 145 on flat ground in cool weather at age 40 for a 13 mile run which from what I understand is the right pace.
I do it because my sanity needs it. Been through a very painful heartbreak after having the guts to fall in love again. Now back to square one again. But I want to get better and love again. and again.
So to sum up, running is way way better than doing nothing. More than 40 miles a week may have not only diminishing returns, but negative consequences.
I'd guess most people here don't realize what a crazy time commitment running 40 miles a week actually is. Even if you can pull off 6 minute miles (4 hours/week), there's still a need for rest after the run, more sleep, more food, replacing shoes and clothes as they wear out. The extra time adds up.