I have been running for many years, since 1988. Years ago it took me a long time to increase my running distance. I had to work into longer distances very slowly or I would be in a lot of pain. I don't have that problem so much anymore. I am still careful. I will not go out and run 13 miles if I have only been running 6 miles in my training runs. I was worried when I ran the marathon in 2008. My longest run before the marathon was 21 miles. I had been running a lot of miles during the weeks and months leading up to the marathon. I was worried because I had never made that big a leap in distance before. It all worked out fine. I was not sore the day after.
After all that I am just saying work into new territory slowly.