When you get to the end of your next 30 miler, think "Could I repeat that 3.33333 more times?". If it took you 2 hours to do 30, could you sit in the saddle for 6+ hours? As previously mentioned, it's not necessarily the physical exertion (legs)...it's the arms, the back, the neck, the feet, the butt, etc. Sometimes it's the clothing...you find out that your socks have a seam in them that didn't bother you at 30 miles but after 60 you've got a nasty blister...or your jersey is chaffing you under your arms. I would recommend going further than 30 at least once before the event...even 50 would give you a better feel for things (and then you only have to multiply by 2!).