Sometimes you'll want to combine workouts, such as when you're planning days off and there's 7 different things I've suggested for each week.
Marathoners like putting the fast continuous run at the end of their endurance runs, so they can work on pace when already tired. Ultramarathoners sometimes put a sprint every 20 minutes or every 2 miles, to change the stress and lower the monotony.
Trail runners and those frequently injured like doing their interval workouts on hills. Running uphill decreases impact forces on the legs and running hard uphill causes one's heart rate to climb quickly. The literal downside is that you have less control over how long your recoveries are. The great miler John Walker used to do almost all of his training uphill because of injuries; his coach would drive him to the bottom of the hill, decreasing the recovery - but not many of us have someone who'd do that for us.
A number of coaches like having athletes do strides and drills before interval workouts, as a warm-up.
The biggest problem with combining workouts is that one generally ends up doing nothing well.
There are countless reports on how to "most efficiently" improve some aspect of training, but they all are problematic. They tend to be 12 week studies and you probably plan to be running a year from now, not just in 12 weeks. If you injure a muscle, you can find studies that show which exercise maximally stimulates that muscle and you might think that that's the one you should do - but you're not training to maximally stimulate one muscle.
You can combine all of the workouts I described into three workouts and there are 3 day per week training plans... but try to name anyone who successfully followed a plan like that for a year, much less several years.
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