First off, my primary foundation is to include a balance of strength, cardio and balance/mobility in my workout schedule.

My cardio is primarily running but I also try to include walking whenever I can. While walking is also cardio I am doing that more for active recovery for my legs as well as getting my step count up. Such casual walks also serve as my zen time unless it's an opportune time to add a workout and I walk hills at a brisk pace, at which point it really is intended as a cardio workout and not those other things.

Strength is typically resistance work, but can also come from running in the form of hills and sprint intervals (for lower body/core) and also from isometrics which are part of my balance/mobility routine.

Balance/Mobility is stretches and isometrics to both loosen me up, and to develop strength in weak spots which otherwise left undeveloped will just eventually lead to aches, pains and injury, particularly from running.

I usually set my exercise program up in 6 week cycles unless something prompts me to change sooner, so it changes fairly regularly. Sometimes the next change is pre-planned in a set of cycles, sometimes I just need to change it to increase or decrease total effort as needed. The reason for 6 weeks is that it takes about that long for your fitness to fully adapt to an increased load making it the new normal, and as long as I'm not overloading myself I can go that long at the new load and then take a "down week" to recover.

That said, I don't take the same approach and simply add more of the same with every 6 week cycle either. Instead those cycles have an overarching goal of developing either strength, endurance or cardio.

For example, from March through early-June I was working on endurance or "base building" which was primarily easy effort running (conversational pace or HR Zone 2) with only one run per week at a mildly faster pace (can still talk but breathing in middle of sentences or HR Zone 3). The idea was to increase my running time/mileage over those 3 months, as well as increase my pace (go slower to get faster). I complemented that with 2-3 days/week of resistance training for strength (2 upper body days, one optional lower), and 6 days/week of brief (about 10 mins) balance/mobility.

After that I switched to a strength focused cycle that had faster running workout days including threshold runs (Zone 4), speedwork intervals, and increased strength training to 5 days/week, and increasing time and reps on the isometrics in my balance/mobility routine.

My new 6 week routine is cardio focused, so reduced strength sessions and balance/mobility and replaced with a 2.5 hour long run Sunday, threshold run (Zone 4) Tuesday, and another workout run on Thursday (either hills or 90 minute long run). I'm having trouble missing a number of these workouts the last couple weeks though, so I won't likely get as much out of this cycle as I had hoped, but it's OK. The downside is that typically the first week or two of a 6 week cycle has me pretty tired but then I feel better until I'm due for that "down week", and by missing some workouts it just extends the adaptation meaning I'll still feel tired in the later weeks in the cycle.

If I find myself in a rut I look over when it started, try to identify what brought it on, and adapt. Sometimes that means having to change my schedule from what I had planned. In the worst cases, when my last option is just taking a break, I'll take a few weeks off. I hate that though because once in a while it's hard to start again, but it is necessary once in a while.