How can I speed up the rare marathon sex session?

Your words: "Normally when my girlfriend and I have sex, I usually last anywhere from 5-25 minutes not including foreplay."

My response: Wow. You are a sex god. Some guys can't get past 2 minutes during penetration time. My guy is like you in that he can last that long. Lasting that long gives her an opportunity to orgasm more than once, if she was sufficiently stimulated before penetration began. Women are capable of having more than one orgasm, and some will do it before the "main event" and then during it.

Your words: "Sometimes though, we have a session that will last anywhere from 45 minutes to 2 hours of straight sex (changing positions is included in that time). I don't often due any slow and steady rhythms either, usually I go at a very intense pace that usually has me changing positions after 5-10 minutes so I can catch my breath or get over the burning in my muscles."

My response: If this is lasting longer than you want to, let her have her fun and get her orgasm(s), then switch to a position that will get you off faster. Cowgirl doesn't really do anything for my guy. However, he can finish faster in doggy. Also, try something tried-and-true that will get you off faster but isn't so tiring for you. When people switch positions, they basically have to reboot, so to speak, so the session can take longer.

Your words: "This usually isn't a problem, but last week, we were expecting a session to last around 10 minutes, and we started out in the missionary position, then when I got tired we moved to doggy style standing up leaned over the bed. Then we went to cowgirl for a good 20 minutes or so with me adding to the rhythm whenever I could, and the rest of the time letting her set the pace. We were having sex for over an hour and a half before she needed to go to work and I didn't get a chance to finish because we were already running late."

My response: I am wondering if you and your girlfriend get sore after an hour and a half of sex. I'm fine with two sessions at different times of the day, but one session of an hour and a half would make me sore.

Your words: "Does anybody have any advice for when something like that happens? Is there anything I or she can do?"

My response: I'm thinking that if you want to finish faster, as you needed to do that day, you could try thrusting faster and letting her maximize your pleasure by adding stimulation of your scrotum to your activity. Hopefully, you don't have problems with delayed ejaculation, as some guys do. You don't mention that this is a problem for you, so I'm not sure.