Your bag will be checked through to PIT at LHR. You will arrive at Terminal E. You will clear immigration, collect your checked bag, and clear customs. You are then legally in the U.S. The first thing you do is re-check your bag at a clearly marked desk right outside.

You then exit Terminal E and cross most of the airport to Terminal B, which AA uses. There is a shuttle bus, but since it goes in the wrong direction (counterclockwise, the longer way between those two) and you have no mobility issues, you don't want to use it. Walk out the front door, turn left, and go past Terminal C to Terminal B. Check the signs in B to make sure you go to the AA side: Terminal B is effectively two terminals separated by a parking garage. Clear security and board there as you would at any other U.S. airport for any other domestic flight.

If the weather is bad, an alternate route is to take the escalator up to the parking garage walkway in Terminal E and walk to Terminal B through the central parking garage. It's not much longer.

I would personally feel comfortable with 1:40 for the whole process, especially since LHR-BOS schedules are padded so you're likely to arrive early. If you encounter unusual headwinds (the usual ones are taken into account) the padding may go away, and if there are delays at Heathrow in addition, you might be late - so have a Plan B in mind, though you probably won't have to use it.