Advice on Saying No to a Running Buddy

OK FitSugar users, this fellow reader needs your advice with a sticky exercise situation.

Dear Fit,
I recently moved from the city to a new house in the suburbs, and was excited to run in my calm, safe neighborhood. There are tons of quiet streets, hills, and even woodsy trails, so I never get bored. I made friends with someone on my street, and it turns out, she likes to run too. We made plans to run last weekend together, and it was fun having a partner to chat with, but I realized I prefer running by myself. Not only does it give me some much needed time to think and be alone, but my new friend and I are on different fitness levels. She has a hurt knee, so she takes it easy, runs slower and for less time, and can't do hills. At the end of our run, I was annoyed that we hardly broke a sweat, but she was really excited and said we should run together all the time. I really like having her as a friend, but not as a running buddy. How can I say no without hurting her feelings?
—Running Solo

Can you help a fellow FitSugar reader out? What advice would you give her?