As it turns out, October weather for the Boston Marathon isn't all that bad (one may argue better than April for several reasons)! One of the nice features for runners will be a fairly high relative humidity, meaning the weather won't be exceptionally dehydrating. The rest of the week looks incredible. Full details in the forecast video.
Today (Monday): Clouds mix with sun. Highs near 70. Northeast wind 4-12 mph.
Overnight Monday Night: Clouds fill in, pockets of late fog. Lows in the 50s. Nearly calm wind.
Tuesday: Early clouds/fog to fair sky. Highs 70-75. Light and variable wind, sea breezes.