I started C25K this year. I'm running a half this Sunday. It will take me 3 hours but I know I can do it. Fortunately, we have had lovely fall weather and I have had no opportunity to run in a pair of compression running tights that I bought. Sunday's forecast is low of 36 with 40% chance of rain. Had low 50s and sprinkles a few weeks ago and managed that without the pants (wishing now I would have wore them anyway). I have seen many suggestions not to run in untested gear. I could do a mile or 2 in the pants this week to make sure there are no weird rubs, but I am not going to be able to mimic the temp. I can take an extra shirt/gloves etc and shed them if its too much. Cant do that easily with tights.
As a follow up - I am more concerned with the rain at that temp than the temp alone. If this was a dry 36, I would be less worried running in a compression short with my knees and shins exposed.
I've been in any number of races where gear, often expensive gear, gets abandoned along the course. Regarding your tights, I would simply put them on over a pair of compression shorts and take them off if things get too warm for you. The good thing about tights is that instead of abandoning them, you can tie the legs around your waist after you take them off.
Fat old man PRs:
Good suggestion by shipo on the layers but definitly go for a test run first. I'm not sure how comfortable 2 layers of compression would be for a half. I have a friend who wore his compression tights for the first time on race morning for a 10K and he said it was a huge mistake. I've done a few races in the cold and rain and you'd be surprised at fast you warm up even in bad weather. Make sure you have some warm dry cloths to put on at the finish. Hopefully the weather forecast will change for the better by the weekend and you wont have to worry about it! Good luck!