The marathon (42.195 km) humbles bad pacing like nothing else. Use the Riegel formula on a recent half marathon or 10K to set a finish time you can actually hold to the line.
Riegel formula from any recent race.
Enter a recent race distance and time.
T2 = T1 x (42195/D1)^1.06 stretches a shorter race to the full marathon. It assumes you have done the long runs; without 30+ km training runs the prediction is a fitness ceiling, not a race plan.
Most blowups come from starting at a pace the formula never promised. Run the first half at predicted pace or 5 seconds per km easy, fuel from 45 minutes in, and the last 10K becomes a race instead of a survival march.