Unfortunately MrsPB its bad luck more and more people will be experiencing. Based on something like 1:16 chance of getting in on ballot if people apply for next 11 years they are only marginally more likely to get a place than not. And something like a third of people won't get in even if they apply for next 15 years!
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