The long run in the first week of training is a relatively easy 6-miler. Each weekend, the long run gets longer, peaking at 20 miles three weeks before the marathon. A tapering period allows runners to gather energy for the race.A half marathon is a road running event of 21.0975 kilometres (13.1094 mi) – halfthe distance of a marathon.Target the Long Run: Every other week, increase your long run by 1.5 miles until you're run/walking 13 to 14 miles. On alternate weeks, keep your long run to no longer than three miles. Your longest long run should fall two weeks before yourhalf-marathon. Plan to take about 15 weeks to prepare for the big day.