You said you did 1000miles in recent weeks, but have you had a rest week at all? I don't mean totally off the bike, but where you do maybe 50-75% of the miles that you did the week before and have at least 1 extra day where you don't ride. I know for a lot of people (including me) their heavy training works in a 4 week cycle - 3 weeks hard and then 1 week easy and it helps me be able to continue the hard training schedule for a long time.

As for hanging in with the bunch - sit on, sit on, sit on! Sit on the back until you are past the bonk point, making sure you eat your Gu while on the bike along the way. If you can eat some of the Gu before you bonk this might help. If anyone asks why you're sitting on, then tell them why and hopefully they'll be ok with it.