HA! Well, I signed up to do an Ironman (IM)... before I even did my Half Ironman (HIM). I wanted to "see how I did", but unfortunately I had to sign up or else the registration might have closed.
When I signed up for my HIM, I hadn't run more than maybe 5 or 6 miles at a time. A half marathon seemed daunting. But I did it! Sure, I got injured a month before... and couldn't really RUN the half, but I did run 2 half's before the race.
Now, here I am... 7 months out from an IM... and I haven't rode more than 100 miles on my bike (only did that once)... and I haven't ran more than 13 miles at one time. A marathon? HA! I can't believe I'm going to do a marathon.
I too have a lot of nagging injuries. Not old age... just that my body rejects running. So, I have taken to walking 5 minutes and running 2 minutes for anything over 5 miles. I am also focusing on speed walking. Right now, I can walk a 13:30 minute mile. I hope to get that down to a 12 minute mile come race day!
Moral of my story... do what you want to do. If your mind and heart are in it, your body will find a way. If you have injuries, look into a run/walk combo, and when you walk... walk fast.
Good luck on the half marathon!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"