I think there is pretty much a consensus these days that stretching BEFORE exercise is not a great thing to do. Sure, there will always be a few people who disagree but overall I think that principle is now well established among PT, trainers, etc.
The "run slowly" advice has to do with building a running base, not with pain. It's part of building endurance, that's all.
When we were kids, we weren't running 3, 6, 13, 26 miles. We ran in short bursts. Totally different.
And there are plenty of complaints about pain during cycling--knee, butt, crotch, etc.
Running actually can be fun. I've seen good, experienced runners on the BG running with complete and total joy, and it's a delight to see. And if you approach it with some care, it can certainly make you stronger.
"My predominant feeling is one of gratitude. I have loved and been loved;I have been given much and I have given something in return...Above all, I have been a sentient being, a thinking animal, on this beautiful planet, and that in itself has been an enormous privilege and an adventure." O. Sacks