As an answer to the original question ("Is my ticker going to explode, or just get fitter?"), with the usual caveat about taking heart health advice from forums vs. a doctor: any exertion you can maintain, but just barely maintain, for 30 minutes is going to increase your aerobic fitness level not cause your heart to explode - unless your heart was going to explode soon anyway.

What you did was essentially a Functional Threshold Power est - the maximum power you can produce across 30 minutes and then de-rate by 5% of so. That's a commonly accepted measure of fitness. If you keep doing that, and the time to make that long climb goes down, your FTP is going up, a good thing. Or, if your time stays the same but the heart rate you observe goes down, your FTP is probably going up - as others said, heart rate isn't that reliable an indicator of actual power output or fitness gain, but is usually good enough.

If you get into it, you can buy a smart bike computer that will let you upload rides to sites like Strava and see some derived power numbers on your rides - not even close to as accurate as using real power sensors on your bike but way less expensive and still useful.