Workplace camaraderie for a new hire is most important than anything else. Unless there is a reason for you to leave work early one or two days, it is okay to stay back and take a short break and leave early, but while you are in the infancy state of you employment, what you will learn from others, who are more senior to you, especially in a relaxed, informal environment, will one day make or break you. Some people love to tell stories. And if there is one place better to tell them than during your long lunch break, it is the happy hour at the local pub. So, I will not only encourage you to keep going to these long lunches, but also suggest not turning down any socializing call coming from your teammates. Such as getting together at the local pub for happy hour, being invited to a golf outing (even if you don't play golf, go to the driving range one day and learn how to tee off, if there is such a possibility), or a barbecue, or a poker night with the guys. I think you've got where I am going.
This does not need to go forever unless you really like these guys. After a few months maybe a year, you will notice the repeating pattern in their stories told or gatherings arranged. Some might be things you like. By all means join the team on those but at that point you will not feel obligated to participate anything and everything. Absolving yourself totally will not be looked cool but, since you will no longer be the new kid on the block, if you miss one conversation, you can catch up on it at the coffee room the next day from someone who was there, because they now trust you.
Also, when you are on these long lunch breaks, or whatever else socializing venues, do not just listen to the others. Try to participate the conversation at a personal level when it is suitable. Tell them stories, especially mildly embarrassing ones (nothing sordid, nobody needs to know your escapades while you were in Las Vegas last year) to make them feel at ease with you. I wish I had that kind of employees at my new workplaces. With the exception of one of my positions, which is the longest one ever lasted in my career, I never had that close knit group dynamic at any workplace of mine.
Enjoy your new job and workplace.