Boston Marathon Runners Soaked & Trash Baggin' in Crazy Rain Storm

Running the Boston Marathon is already hard as hell ... and that's without being caught in a TORRENTIAL DOWNPOUR like the runners in Beantown are Monday!

The field of about 30,000 competitors are gonna have to be Boston Strong to make it through today's race -- attempting to run 26-plus miles through a damn flood and winds approaching 40 MPH.

Race officials also say it's the coldest marathon in 30 years ... with a starting temp of 38 degrees.

It was a pretty crazy scene at the starting line -- athletes fitted in bandanas, ponchos and trash bags -- and it's only expected to get wetter with the rain picking up in the afternoon.