Coming back to running after a long break; what did your early months look like?

Context:
I used to run regularly back in 2017–2018, mostly training by feel. At that time, my PRs were roughly:

• 5K: ~24 min
• 10K: ~50 min
• Half marathon: ~1:55

Fast forward to now—I restarted running in December 2025 after a long break. This time I’m tracking heart rate using a WHOOP and trying to focus on Zone 2 training.

What’s been a bit humbling is how much my fitness has dropped. Right now:

• Most of my runs are walk–run sessions
• Even with very light running effort, my heart rate spikes within 1–1.5 minutes, forcing me to walk to bring it back down
• Long runs average around 8:45 min/km
• On threshold days, I can barely hold ~7:15 min/km for 1 km before completely blowing up

It’s honestly hard to reconcile this with where I used to be.

More context:

• Weight in 2017–18: ~72–73 kg
• Current weight: ~80 kg
• Height: 177 cm
• I’ve had COVID about 4 times over the past few years (most recently in November)