"How to Run a Personal Record: Cover the Ground in Front of You Faster Than Ever Before," by Dave Kuehls, pg. 16-17.
The Saturday long run carries over from the Road Work phase. If you are involved in 5K, 10K, or half-marathon training, the long run will stay constant or will vary slightly from week to week. For instance, one Saturday you might run 12 miles and the next Saturday you would run 10 miles, and then next you will run 10 miles again (for half marathon training). If you are involved in marathon training, the long run will still be building during the Strength Work phase. For instance, a Saturday long run will go from 14 miles to 18 miles in a one-week period of the Strength Work phase (marathon-training plan). Carry over all the long run lessons learned during the Road Work phase. And make sure not to skip it. The long run at this point is what gives you the endurance to handle your hill runs and your tempos runs. Without a long run for a period of weeks, you will simply "run out of gas" one day on the hills or during a tempo run. And then you will have to go back to the drawing board - back to the Road Work phase to build up your endurance again.
Note: The long run in this phase is not treated exactly the same as during the Road Work phase. You don't have two days to recover and prepare for it after the midweek run. And you don't have three days to recover from it before the midweek run. Instead, only one day separates your Thursday tempo run from your long run at the end of the week, and only two days separate your long run from your hill run on Tuesday. This means that the recovery days serve a slightly different function during the Strength Work phase (see earlier post re: recovery days), but also the long run itself has changed. No longer is it the ultimate workout for the week - the reason for your week's running - but instead, a piece of the training week that also includes two other efforts that need to be focused on: your hill and tempos runs.
What this means is that the long run might have to be handled a bit gingerly in the first week or two of this phase: Start slower. Spend more time in the warm-up phase of the long run, the first couple of miles when you are getting your legs, and also resist the temptation to push it the last couple of miles.
What it also means psychologically could be just the opposite. That is, that the long run, since it is not the sole focus of the week anymore, could also become easier as the Strength Work phase goes on. There is less pressure on you to complete the long run. Instead, it is simply another workout that needs to be done during the week. Another factor making the long run easier at this point could be the fact that you are getting "more fit."