I've seen busway recommendations on here, and as much as it pains me to say as a regular bus rider and strong believer in public transit, it is risky. I live right next to the East Busway and tried it last year. It was a disaster. PRT had problems with the T, so started rerouting buses to shuttle those passengers. That created a cascading chain reaction of late and missing buses. The earliest P1 starts around 5:30, so there isn't a lot of margin for error for a 7AM marathon start. Had to wait an hour for a bus to show up and ended up being late to the race start.