Running in London and marathon training

Hi everyone, I'm relatively new to London (again) after years in nyc and other countries. Thought I would try asking a few questions on this.

I'm getting back into running and I'm about to move just south of regents park area. I signed up for a Protected content so I'm interested in finding a running group. Any recommendations for run groups or marathon training? ( I've run several in the past). I would love to meet a few runners! (Not super fast). Anyone think it's safe in regents park early in the AM? I'm used to running in Central Park.