Susie, I don't know if there is an answer for that question. I've been running for 3 years (I'm 33 now), and it's still not easy. When I was 20 lbs lighter it felt easier than it does now, but it still was never easy. 2 marathons (getting ready to do another) and one IM later, I still don't love to run. I do it because I have to, not because it brings me great joy and because it's easy. It seems like most runs are a struggle and I hardly find myself getting in the "groove." Don't know how much of that is mental, tho (probably a lot).
So- I gues I don't have an answer, but I do know it does get easier. It's certainly easier than when I first started, but I'm just not sure in what measureable amounts we're talking about.