Did you consider a simple solution like a Polar HRM? Although best known in sports training applications their
website also talks about ‘protective services’ – fire, police etc – so it’s not all about getting your fitness up, but when you’re not using it for your test purposes it could come in handy in the gym or pounding the pavement.
The higher-end models have data download, which would give you your timeline. My modest one doesn’t do that – I just eyeball it. It has also been my everyday watch for the last eight years.