My answer may surprise you. I believe 20 miles is actually too far to run in training. There is no such thing is an easy long run, at any pace. The stress to the body during a 20-mile training run can be straw that breaks your body. The 20-mile training run usually comes late in the marathon training schedule, a time when all systems are taxed to the max. This is a time you are most susceptible to sickness and injury.
1. Alternating Hard and Easy
Hard days are long or fast runs. Easy days are short, slow or ones spent in the arms of a LA-Z-BOY. I suggest at least one day off each week but you could miss all the easy days on the schedule and still do just fine. I don't suggest it.
Sweat First!
Ideally you should stretch after you break a sweat. There is risk of injury when stretching cold muscles. Running about a half mile very slowly then stopping to stretch is ideal. During winter this is not practical so if you stretch before you run, be gentle. After the run, schedule 2 to 5 minutes to stretch.
Often ignored but something to think about. Yes, you can change your form. Natural form is a myth. Some quirks you may have are ways your body deals with asymmetric differences. They may be necessary, but good form can be learned. Here are a few things to try on your next run.
