Step 1 Calculate your max heart rate. There are several painful methods for calculating this number. I have found this calculator by Gallagher fitness, is easy and very accurate.
Step 2. Multiply your max heart rate by .85
My max heart rate is 175 so.85 x 175 = 150
Step 3. Go to the track or measured mile and run two miles as close to this number as possible without going over. If you go over this slowdown.
Step 4 What you average for this 2 miles is your marathon pace.
Step 5. Now when you run your marathon you can either wear a heart rate monitor and do not go over this number calculated in step 2 or keep your pace as close to this pace as possible for at least 15 miles
I like to perform this test two weeks before the marathon
List of my science of running tips
Calculate your marathon pace
Correct arm swing while running
Drill prevent heel strike
3 ways to run faster