LogicScience... the link you post states a view that is completely contrary to the one you explain in the text! (to be fair, I would also have guessed that my Garmin did take account of slopes... but it seems that Mr Puffy and cougie are correct.) This is a quote from the strava article.
GPS-calculated distance will "connect the dots" between your GPS points and triangulate the distance between the coordinates. GPS-based distance assumes a flat surface and cannot account for vertical speed, or the 3D velocity vector that would take into account the increase in distance with topography. However, the effect on topography for GPS-calculated distance is minimal - for a 10% grade, distance would only increase by 0.5%. For a 20% grade, distance would increase by 2%. This explains why GPS-calculated distance can sometimes be slightly shorter when compared with ground speed distance from a wheel sensor.
As others have said, skip park run. Run a couple of times this week... but not after Friday... or maybe not after Thursday, as you plan.
It's not going to be warm for the race, so keep a family member close to the start so they can grab your jumper off you. Or alternatively, wear a throw-away garment over your kit, and discard at the last minute.
Comfy clothing. Bit of vaseline on your feet, between your thighs, on your nips, and anywhere you think might rub.
Set off noticeably slower than you think you can run. When was the last time you exercised without a break for over an hour? If you set out slowly, with determination to jog all the way, you might do it. If I were you, that would be my aim. Slow. Steady. No walking. But if you need to walk, then do.
Obviously you got a bit of stick for underpreparation earlier. And you deserved it But you live and learn. Hopefully you'll catch the bug. Enjoy the day.
Just to back up what others have said... weight will play a big part in your success and take huge amounts of stress off your shins.
Also... don't practice the bleep test again more than once every couple of months over the next 6 months.
Couch to 5K is a great way to go about it. If your shins are really hurting though, read all about shin splints - which are very common in people starting running - and which can soon become a stress fracture if you try to plough through it without care.
9 months is both a long time and a short time! 9 months is plenty of time for you to achieve this (and frankly, if you do not, then you may not be ready for the discipline of the force - unless there is a very strong reason why you don't succeed... like a long time out through injury). But 9 months will soon go... so be disciplined from the off.. but that doesn't mean go racing off too fast. Steady away will get you there.
With 13 weeks to go, changing plans to another one aimed at your current running ability (but perhaps a little more intensive) would be no problem. But there really is no need to panic. You could also choose to stick with the current plan, but throw in an extra run for as many of the remaining weeks as possible... even if it's a 3-6 mile recovery pace run, it will make a big difference. It's that latter route that I'd recommend... but if you can also "lose those pounds" at a steady, sustainable weight, then that will make a great difference too.
But you are just having a crisis of confidence (common, but in your case, unnecessary)... because the times you're running are great. Surely you can see that a 2:22 half marathon, 15 weeks from race day, shows great promise. And 2:38 in a half that includes 6 miles on a sandy beach... likewise.