I've paced a lot of groups in marathons and my guidance is usually don't worry about mile 1 being 20-30 seconds slow...between getting through a crowded start, and not being warmed up, etc. you can recover from that. I echo what someone else mentioned - if you can note where the half mile mark is, you get a data point earlier. After the first mile, start working your way down to your goal pace and that SHOULD feel easy - not like a recovery jog, but not taxing.
It sounds like you're targeting 8:00 so if you can get through the first 4 miles under 33 (8:15 avg) you are perfectly fine. You can make up a few seconds here and there for the next 22 miles - lean forward a bit on a downhill and get a few seconds, give yourself a mile or two where you open it up a bit to like 7:50 and that 1 minute goes away pretty quickly.
Run slow when you SHOULD so you're able to run fast when you need to - and "fast" may just be maintaining the same pace after 20 miles but it's hard now because you're tired!
Bear in mind you will also have a taper so goal pace should be comfortable come race day. A light workout I try to do the week of a marathon (like Tues or Wed for a Sunday marathon) - go to a track, run a warm up and then run 400m repeats alternating 5k pace and marathon pace (no recovery between). Ideally, you will find it's difficult to run slow enough to match your marathon pace. 5k pace is hard, and by comparison marathon pace feels quite easy. Don't go nuts with volume on that - maybe 1.5-3 miles worth of 400's MAX. That's a workout that I feel is more for my head and confidence than anything else - you get some race week nerves out by doing a light workout, but it's not so taxing that you're tired afterwards. And if it goes well, you come out of that workout thinking just how easy marathon pace will feel early in the race.