build down running?

To finish a full marathon within 3 hours I have to run as pace running or build up running. Today I started running as a pace running 4:20~4:50/km, but I had been exhausted and ran as a build down running...... It was a result of short of training!


Training: 29km (25km pace run & 4km cool down)
monthly mileage for September:　322km(monthly target350km)