Most watches with a HRM function will have an indoor setting of some kind that will record the heart rate pattern and potentially calories burned during the exercise. Some will record steps etc as well but that is less relevant I guess.
Agree its mostly the shorter stuff that will affect real pace and the longer runs are mostly for endurance. Efforts faster, long runs slower.Also agree there is no harm in doing the last 1/4- 1/3 or so of some (not all) of your long runs at race pace.
With a 1:29 half (albeit in a marathon) a 3 h marathon will be a big step.Good luck