I've been running a particular number of miles for each run. When I'm tired I run for more minutes because my pace is slower, and when I feel good my run takes less time due to a faster pace. That is just the opposite of what I should be doing to protect my body from injury. When I'm tired, I should be running fewer miles, and when I feel good I should be running more miles. I've decided to return to running for a certain number of minutes, referred to as running time, instead of a certain number of miles. In doing this, I'll give my body more protection against injury. If I'm tired, I'll run slower and do fewer miles, and if I feel good I'll run faster and do more miles. This is discussed in more detail in my training site.
Reverting to a time-base means that I'll have to change my goals. Instead of having goals for certain number of miles, I'll have goals for certain number of minutes, as follows. The specified minutes are minutes moving not total minutes. The total minutes is longer if I take rest breaks.
Rest run: 1 hour (60 minutes)
Medium run: 1 hour 40 minutes (100 minutes)
Long run: 2 hours 20 minutes (140 minutes)
These new goals correspond approximately to my previous goals in miles.