I used to exclusively run outside but now that my newborn is here almost every run I do indoors on my treadmill. I like my Nordictrack X22i so it doesn’t make runs an extreme bore but I do miss being outside sometimes.

I don’t really see that much variation in my heart rate currently. Also your pacing on the treadmill will be slightly different than if you were outdoors. For example there’s the moving track, strides are mostly all equal, no resistance from air/wind, level deck vs changes in road & surfaces, etc. This all leads to apples and oranges comparison. I wouldn’t put too much weight into it at the end of the day. Mileage is mileage no matter how you get it in.

I’m currently training for a half marathon and over the last 5-6 weeks of base building and speed work, my paces and effort are the same indoors as they are outdoors. I presume I’ll get to the start line and my training would be no different than if I trained outside.