You only have to finish 1 marathon before running Boston, you just have to run it fast enough to qualify. <a href="http:&#x2F;&#x2F;www.baa.org&#x2F;races&#x2F;boston-marathon&#x2F;participant-information&#x2F;qualifying&#x2F;qualifying-standards.aspx" rel="nofollow">http:&#x2F;&#x2F;www.baa.org&#x2F;races&#x2F;boston-marathon&#x2F;participant-informa...</a> You can also get in with a charity slot without qualifying.