There's no reason to feel guilty! Everyone understands where you're coming from. I thought it might be good to double-check your pace (which you did) so you didn't line up at your 10k race in the "10-minute mile" starting group and get trampled! Whether you're fast, slow, or middle of the pack; everyone gets the same cheer from bystanders. Soak up the atmosphere; your first race is always memorable, and a ton of fun.
Whatever your pace, walking or running, you are out there doing something, moving. Your goal of completing the 10K is a great one. My hubby and I did our first 10K this spring, walking (it was pouring down rain) and were proud of our accomplishments. Believe me, there were plenty of people ahead of us but we weren't last either. Your time is your time and after your first race, you'll have a baseline for future walks or runs. Have a wonderful time and again, good for you for getting out there!