yo yo yo wrote:
If I was running 3 days a week, would it be OK to do a 15 miler if my weekly total is 30 miles...?
If you look at how many marathoners trained before World War II you'd find a lot of them, maybe most, who just ran 3-4 days a week. Something like 15, 10,10, or 15,10, 5, or maybe 20, 10, 5, was pretty common. In those days a lot of people believed that running was too stessful and doing a lot of it was bad for your health so a lot of people really limited how much they did.
I don't know that there's an across the board answer to your question. It depends on how much of an effort that 15 mile run is for you. You'd fit right in with a lot of marathon runners from the 1920s and 30s and obviously for many of them this was no problem. I think you'll find your answer by trying it and seeing how you manage the other two runs. If you're able to do gett runs like you want to most times you're probably okay but if you struggle with them frequently then the 15 is probably too much.
I don't know if you've been around the sport long enough to know who George Sheehan was but he was a pretty good masters runner and well known author. I believe he was the first person older than 50 to run a mile in under five minutes. He was also a decent marathoner, not great, but very respectable, best time was 3:01. At some point he thought he wasn't getting the results he wanted on a 6 or 7 day schedule and went to three 10 mile runs a week. He managed to run respectable marathon times on that schedule.