As much as I'd like to give you a good idea, I'm not sure I have enough information to say whether it's possible. Doing a 5k at HM Tempo (about 28.5 min in your case) is a mildly difficult workout, but pales in comparison to 45-60 min of continuous HM effort. Although a 2:07:48 is 9:45 pace not 9:30s. Someone aiming for a 2:07:48 HM Tempo (9:44 pace) would do about 15k racing at 9:30s. I'd say 5k of just slower than LT (9:23) and right around 15k pace is right in the area of a good tough training run. So while you were slowing yourself down during the training run itself, like you said, it doesn't really paint a complete picture. I would advise though that in the future you don't try and do a test like this any closer than 10-14 days out from the event. Little is gained physically from the effort, and you may be hampered by the recovery. As I like to say, "Save it for Race Day!"
In a more general sense, these are the things I think about when coming up with race day goal pace:
First, what is my motivation for this race? Is this an all or nothing proposition? For instance, in your case it appears like you're approaching the race as a 2:07:48-ish or bust. If you get 2:07:47, then success. If you get 2:08:00, then failure. If this is the mindset entering the race, then you have your goal pace in mind regardless of whether it meets the standards of actually being accomplishable. You go at that pace and hold on for dear life throughout it on the hope you can maintain it. But if for instance the approach is, "I want to do my best, whether that's a 2:07 or a 2:09. A 2:09 would still be a PR over my 2:11." Then that's when you use other information to help set goal race day pacing. How much risk am I willing to incur? The faster the goal relative to my fitness, the higher the risk. Am I ok with a 10% chance of 2:07 and 90% chance of 2:15 vs a 30% chance of 2:09 and 70% chance of 2:12 (made up example)?
Secondly, I evaluate the race day weather and course. The further away from ideal that the weather is going to be, the less likely I am to hit optimal fitness performance. I can still do my "best" on that day under those particular circumstances, but it may pale in comparison to my best on a better weather day. So for me, if the T+D (Temp+Dew) is under 100, then I'm good for a max effort. If the T+D is between 100-120, I'm ok, but may not reach max potential. If the T+D is over 130, then I know I very likely will not be able to reach max potential. So be realistic with the effects of weather (be it T+D, wind, solar radiation, etc.) and its effect on your racing ability. The course matters too. Because you could do a time trial on a perfectly flat and beautifully cold crisp morning, but then race day is up the side of a mountain on a crazy hot day, and whatever the time trial said is going to be less relevant. I like to use Grade Adjusted Pacing (found on Strava or Garmin Connect) for planning out how each mile's pace will be affected by the change in elevation. So let's say you were aiming for a 9:30 pace on an out and back course. The first half is all uphill and the second half is all downhill. If you attempted to run 9:30s as an even split, then your effort in the first half (uphill) is likely going to feel much much harder than 9:30s. As the race wears on, the HM Tempo pace, may feel more like 10k effort. Despite having a downhill second half, you're likely not to make up the difference based on the hole you dug yourself. So basing pacing off of each individual miles change rather than an even split approach can be useful depending on the course design. The elevation is probably less relevant a consideration in FL though.
Lastly, once I've decided on my motivation and evaluated the course/weather is when I assess where my current fitness is. Have I done anything in training or racing in the last few months to suggest my goal time is reasonable? You're aiming for a 2:07:48. Can you/have you run a mile in 8:17, 5k in 27:47 (8:47), or 10k in 57:44 (9:17)? I'm not saying go try this out, but do you have reason to believe you could achieve these times? The more confident your answer, the more likely a 2:07:48 is within your wheel house. It's important to remember that when you have a specific goal time to allow some watch GPS buffer. You've been training by the GPS watch. When you run 3 miles on your watch you did it in 9:30s pace. But being on the race course is different. You're unlikely to run the tangents perfectly. So you're unlikely to run 13.11 miles per your watch on a perfectly measured course. I prefer to look at old GPS watch data from runners in that same race in prior years. Did most do 13.15 miles, or 13.3 miles, or somewhere in between? I use that to set what I guesstimate to be my final watch GPS distance and then set my very specific pace goal to it. For instance, a 2:07:48 at 13.11 miles is 9:45 pace, but at 13.3 miles is 9:37 pace, and 13.5 miles is 9:28 pace. So having a good idea for what type of buffer you'll need based on watch GPS is important if that's going to be your main focus when making in-race pacing decisions.
Regardless of what you choose, my number one advice when it comes to racing is to treat the race as a victory lap for what you've done and not an assessment. Keep your motivation high. I'm a big believer in the Psychobiological Model of Endurance which boils down to motivation influences our perception of effort. If we lose motivation, a task may not be any physically more difficult than it was when motivation was high. Yet the perception of the effort of the task will increase thereby making it seem harder. Stay highly motivated and you can maintain a similar level of perception of effort.
. I really appreciate this @DopeyBadger and yes, I will reach out if needed. I can't thank you enough!!
