This is long, I apologize. I obsess over this stuff and just love it. I promise if you read this, you will have your socks blown off...eventually.
I agree with a ton of stuff that's been said. I experimented with high mileage earlier this year. I found CONSISTENCY to be huge. BUT I kept increasing. I was feeling aerobically SO GOOD at 60-65 miles per week. I wish I had stayed there for my training cycle (6 months) and then increased a little more during the following cycle to like 75-80 mpw. Instead I felt so good, that I kept building to 90-100 mpw until I ended up with two hurt solei muscles. I was doing intense marathon training and all the best marathoners do high mileage. Even though I'm 29, I hadn't consistently done more than 50 mpw at any given time. The guys doing 120-150 mpw built up the ability to do that over YEARS. Sure, there are always exceptions. The guy who doesn't get lung cancer after 40+ years of smoking. Then there's my grandma who died of lung cancer this year and didn't smoke a single cigarette--my grandpa smoked a ton. Always gonna be exceptions.
The answer to most of life's questions is "it depends". Should you up your mileage? It depends on...
1. How consistent have you been in your training? Has your mpw been consistently 35-40? Or have you been up and down, some weeks 1 run, other weeks 40 miles?
2. When you warm-up, cool-down and do your 6-8 mile runs, are you starting out slow and gradually speeding up as your body suggests? Or are you running pretty much the same pace all the time?...fast.
3. How many workouts per week are you doing?
If you've been consistent. If you're starting out slow and running smart, I think increasing 1 mile per workout done each week up to a max of 10 (if you're doing 10 workouts a week) per 3-week period is fine and then paying CLOSE attention to your body and making sure you feel good in every way. So assuming you've done 3 weeks of 40 mpw in a row, and assuming you're running 7 workouts per week, then I would increase to 47 mpw for the next 3 weeks and see what happens.
THE ULTIMATE BEST THING you can do is read. READ. READ and READ some more. Letsrun is a great forum, but everyone is different and everyone has opinions based on what they or so-and-so did. That works. That's great. You however have a different body, different blood-type, different dietary needs, maybe you're done growing, maybe not...and the list of factors and considerations goes on and on. Read, research, plan, apply, repeat. That's my recipe for awesome.
I live in Provo, Utah. I did an internship with the BYU Men's Cross Country team. Say what you will, but they consistently pull out top finishes and the majority of their runners are from Utah. I also work at Timpanogos High School in Orem. They took 12th overall at Nike Cross Nationals. I also talked at length with Connor Mantz's (top Utah runner and Foot Locker beast!) coach on a bus to Foot Locker Regionals--all these coaches/teams are using some form of Daniels' Running Formula mixed with other stuff. I also have loved John Kellogg's unpublished book of running wisdom compiled by another guy.
Here are the great running resources I have compiled:
Books:
Daniels' Running Formula by Jack Daniels (look for it on Amazon.com)
Training Wisdom of John Kellogg (use the following link to get to this free book:
http://distancegoldmine.weebly.com/uploads/1/5/2/0/15207232/jk_collection.pdf
)
Running to the Top by Arthur Lydiard (amazon has it, not expensive)
Road to the Top by Joe Vigil (read the comments on Amazon.com to find out how you can get the book for super cheap)
Running with Lydiard by Lydiard/Gilmour
Born to Run by Christopher McDougall
Thrive: The Vegan Nutrition Guide by Brendan Brazier
More Fire: How to Run the Kenyan Way by Toby Tanser
The Greatest: The Haile Gebrselassie Story by Jim Denison
Tapering and Peaking for Optimal Performance by Inigo Mujika
Factors Affecting Distance Running Performance by Steve Magness (free book somewhere on this site:
http://www.scienceofrunning.com/
)
Websites:
Letsrun wouldn't let me post the websites because it said it was potential spam...keep searching man.
Never stop learning. Never stop searching. No one has all the answers, but there are some who get pretty dang close.