Once you get over the hump and develop a certain amount of cardiovascular fitness, it stops being unpleasant and stressful.

The real problem is that most people don't feel like this is true. It really takes a solid 6ish months of earnest effort (AT LEAST 3x per week, probably more) to develop cardiovascular fitness. For some people, it'll take even longer.

I run an average of 6 days per week for the past 10+ years. At this point running is just about the easiest thing I do, it doesn't take any mental fortitude at all to do it. It wasn't always that way though, I used to dread it.