Believe it or not, if your long runs are up to three miles, you’re ready to train for a half marathon. Running 13.1 miles might seem impossible now, but you will make it, as long as you take it one week—and one mile—at a time.
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 your half-marathon. Plan to take about 15 weeks to prepare for the big day.