HRE and fred - Thank you for the responses. It is utterly fascinating to me that this was the norm back in the day (among dedicated runners, at least). It's also encouraging that if I need to do this due to scheduling contraints, it can be done.
When you and guys you knew were doing this kind of mileage, were they getting injured a lot? In general, would you say that injury rates were greater, lesser, or equal to today?