GTF, right. I glossed over the “maximal training benefit” of single runs comment a bit too much. To use myself as an example, I have about an hour to run during my lunch breaks (much like you stated, a required 60 minute lunch break with the flexibility to make it 90 minutes, which allows for time to change, shower, eat, etc.). That maximizes the training benefit, given my time limit, of those runs. Now, to improve my training, I'm left with only one choice. A run before or after work. It's not that making at least a couple of those 60 minute runs into 90 minute runs wouldn't offer benefit, it's that I don't have that option. I've maximized the benefit I can reach from those runs.
Of course, if I was only running for 30 minutes during those lunch breaks and adding in another 30 minutes after work, that would be a waste. I'd be better off doing the whole 60 minutes in one shot. To me, maximizing the benefit of a run is not just doing what would be optimal regardless of circumstances. It's doing what is optimal given circumstances. In my case, it's the 90 minute lunch break. That's my limit. While I might benefit more from making those runs 80 minutes, it's not an option given the job I choose to stay at. Because of that, I've maximized the training benefit available with that run at 60 minutes. Now, to step up the training, I have to add in a second run on top of that 60 minute run.
Rita, good point about thinking in terms of time rather than distance. While everyone talks in terms of distance, myself included, I actually prefer to think in terms of time. To be perfectly blunt, a 10 mile run for you is different than a 10 mile run for me. While I talk of my lunch hour runs being 9 miles, I chose 9 miles because, on the typical day, it will take me at least an hour to complete. On a very good day, it may take me less than an hour but that less than an hour will be at a relatively high intensity. I'm thinking about that one hour time, even though I'm talking about that 9 mile distance. For someone else, maybe that one hour number works out to 10 miles. For yet another, it might be 6 miles.