ultramarathon training

had a 30-minute sunrise run a while ago..the weather was nice the sun was just rising. my plan was to run for more than 2 hours which should be equivalent to 21 kms at my own easy pace, but the pain on my shin had to let me stop and cut my training time to 30 minutes.. the 50 km race is scheduled on June 18.. with my condition now i have to make a new training and game plan.