Sounds like you did exactly the right thing.
Pleased to hear all tests negative; you should take this at face value - it is very reassuring.
In a one-off episode like this it can be impossible to make anything more than an educated guess, or blame a virus.
It may be that something caused your natural cardiac 'pacemaker' to slow down from its athletic usual - a virus can do this.
If you never get an episode like this again, you'll never know what it was, and it'll never be a trouble to you.
Living with that uncertainty can be tricky
At that background level of fitness, with those negative tests, I would also advise gently starting back as you describe.
Initially, for your own sake, and those close to you, perhaps carry a mobile phone if you don't usually, and stay nearer to civilisation than you might normally?