So, when you start running, your cardio/lungs start improving after a mere 2 weeks. Your muscles start strengthening after 1 month. But, it takes a good 6+ months for your bones to get stronger!
I learned this the hard way when I developed a stress fracture early on with my running this year. While I was strong because my bones were carrying a lot more weight than they are now, I was weak because I never did any impact exercise. Simply put, my bones were not dense.
There is a good reason why skinnier old women have a higher percentage of developing osteoarthritis than heavy-set women. They simply don't have as much weight-bearing exercise from normal day-to-day routines. The skinny women who run, however, rarely have osteoarthiritis, simply because they engage in regular weight-bearing exercise. Think of your body as the ultimate barbell....make it work for you, and it will pay you back three-fold!
My bones are definitely stronger. Early on, I had to make a regular occassion of icing my knees (in particular my right knee) after running. I then realized that I only needed to ice it after longer runs (4+ miles). Now, I can run 3 days in a row, and it doesn't hurt! This is a miracle!!! Well, maybe not a full-blown miracle, but a mini miracle nonetheless! Ok, Ok, I know I shouldn't overdo it and run too many consecutive days, but I do love how liberating this can be now that I'm not slave to a calendar...and if I have to miss a day due to a major thunderstorm, it doesn't throw off the rest of my calendar!
So, I'll try to do a few back-to-back days, maybe shorter 2-3 mile runs on a regular occassion and see how I feel. If I falter, trust me, you will know about it...but if I don't complain about it, assume its going fabulously!