Heartbreak hill is the famous lat hill between mile 20-21 on the boston marathon. It is famous because it occurs right where the athletes get tired.
There are many great editorial photo opportunities because there is a lot of straining and exertion. Also, the athletes are running slower(uphill) so you have more opportunities.
To get there via subway, take the Green Line (B branch) to the end of the Boston College line. Cross the street(sunlight will be coming from the left) and walk a mile up the hill and find a place the light is good.
If you want a press pass you need to apply in February.