Working for every second wrote:
You both make good points. I thought that doubles actually put more stress on your body since you have less recovery time. I've also been told that the longer you can keep your heart rate up, the better, and pace doesn't matter as long as you're getting a good effort out of it. Therefore, a 14 miler at 7 flat would help more than an 8 and 6 at 6:20, and put less stress on the body.
If I did do doubles, how often? Would twice a week help at all?
All the doubles vs singles is debatable if its for the same distance, but usually when you're doing doubles you can get in more miles then if you were just doing singles.
Obviously long runs are important, so you'd have to have that 14-15 mile single run once a week. However for your regular days, instead of 9-10 easy, you could do a 7 and 4 or 8 and 3. You'll end up getting more mileage with about the same effort.