I didn't start running until I was pushing 50.
If I'd known then what I know now, I wouldn't have given a toss about being last. Because with years of experience under my belt, I know that it really, really doesn't matter. But at the outset, I didn't know that, and would have minded very much indeed.
I still want to tell you to do it.
I was terminally unsporty at school, didn't believe that C25K could possibly work unless you were one of the sporty girls who has lapsed a bit, though I was the wrong shape to be a runner (excuses and insecurities ad nauseam). But I ended up running quite well as a veteran. Much to my amazement!!
And finding that running communities contain lovely, friendly welcoming people (OK, there are still a few over-entitled weirdos, but really not many). And we all have shockingly bad days out of the blue - I went from one race winning my age/sex category to coming last in one on the same course only 4 weeks later, because the wheels all came off that day and I'm still not sure why. And it was a bit shit on the day - I wanted to tell everyone that I wasn't crap really. But that was my insecurity speaking. Every single runner has had an off day. Everyone has had The Fear of making a fool of themselves. And it takes time, a lot of time, years of time to fully grasp that it really doesn't matter
You don't have to do this race, you know. It's not a sign of character or testament to your ability or whatever. But loads of us do events because it gets us off our arses, running somewhere pretty where we might not otherwise bother to go, hoping to have fun, get a respectable time, maybe see running mates. And 1hr15 is fine as a time. Yeah, there will be lots of people in 1hr or less, but the course will still be open, and absolutely no-one will be judging you for being slow.
As an elite friend of mine put it I'm running my threshold for 40mins - you're doing it for an hour. OK, I've e covered the ground faster, but you're the one making the same physical effort for longer - well done.
I'm not sure now where all this is going! It's come out a bit stream of consciousness! I think I'd say "do it" - other runners are overwhelmingly likely to be supportive - we all have good and bad days, great form and total loss of form
BTW: do you have any running buddies? If being alone at the back is daunting (a totally reasonable reaction, btw) then being with someone else might make it less scary