Runners often wonder how many 20-mile runs are needed before a marathon. This article breaks down the logic behind the classic marathon buildup, shares current advice from coaches and runners, and helps you plan your long runs for the best chance at a smooth race day. Packed with practical tips, it also discusses when 20 miles might be too much—or not enough. If you're curious about how to prep for the big 26.2, you'll get solid guidelines here.