Might consider Blue Line for that trip
If you're looking for the transit option headed towards the TD Garden, the Blue Line might be the easiest option if you're up for a 7-8 minute walk on the other end. Due to the closure of Government Center (Green Line transfer), Bowdoin Station (the downtown end of the Blue Line) is open at all hours now. From Bowdoin Station, Google Maps indicates a 7 minute walk to the HI Express on Friend St. I'd say that's probably conservative, especially if your bags are light. It's about 0.3-0.4 miles.*
The Blue Line isn't free like the Silver Line is at the airport ($2.50 per person ticket, $2.00 if able to get a Charlie Card), but it'll be a pretty quick trip from Airport Station to Bowdoin. You'll have to take the Massport shuttle from the terminal to the Airport Station, but if you take the Silver Line, you'll need to transfer to the Red Line, and then to the Green or Orange to North Station to end up near the HI Express.