I'm making an assumption you might mean bonking. Hitting your lactic acid threshold does not mean bonking, at least with the experiences I've had. Hitting the lactic acid threshold basically means your legs will produce lactic acid more than your body is able to eliminate it. For me the first signal is leg burn. This can occur on a long and/or steep hill or if you're pedaling very hard for a long time on a flat against a headwind or something. When that happens, knock down the cadence and drop to a lower gear to bring down the heart rate; you will go slower but your body will then be able to eliminate the lactic acid and after a few minutes you won't feel the burning in your legs. As a separate issue, to avoid long term fatigue just pace yourself 1-2 mph slower than usual if you're not used to longer rides.
Bonking is a different story. You must maintain nutrition levels suitable for you. If you bonk, it might be hard to come back from - for me I stop, rest, eat, and give it a little time if I'm feeling extraordinarily weary. One time this summer I did bonk. I had to rest/recover for an hour till I got back on the bike. I've done metrics on just gu, but always carry extra stuff like a banana, raisins, and granola bars or something. A prime factor in keeping many people from NOT bonking, is plenty of electrolytes. Keep gatorade or something similar with you on every long ride, even if you think you don't need it.
Eat good hours before, but don't eat "heavy" just before you start. It sounds like you'll be riding with other people. Ask someone around you when the hills are coming up. Certainly someone might know the route before you start - ask them. Do you know the route ahead of time. Go to Google Earth and check elevations or do an elevation profile or better yet, look at a USGS map (www.topozone.com). I never have road maps with me when I ride unfamilar ground, I always have USGS maps (more information).