That's what cutback weeks are for. First, you don't need to get progressively harder every week. Just do the workouts and trust the plan. Second, if you're wearing down after 3-4 weeks of doing the workouts, take a mini-break with a cutback week.
Strides during base building should be fine. Once or twice a week, when feeling good, let loose with a handful of 15-20 second bursts. I often even do these right in the middle of a run using a minute, give or take, of easy running as recovery between the strides.