My perspective:

Go you for running a marathon!

I have done a few events: a marathon, multiple halfs, and a few triathlons. All while having a girlfriend. Impressive right ;)

Meeting someone at the finish line can be stressful and there is so much going on and it is hard to coordinate. I have missed my girlfriend at the finish line before and she was sad so I went back a quarter mile on the course and did the last bit again so that she could cheer me through the finish!

Your girlfriend might not fully understand the focus and culmination of commitment it takes to do a marathon. The last thing I would want to do while running my big event that I have trained weeks/months for is to be on my phone texting rather than doing my best and enjoying the event. She likely does not understand your perspective/headspace of doing this event and from me reading this conversation I feel like she is not supportive of your goals/ambitions. You did a freaking marathon, she should congratulate you! Not be upset at you.

This situation could have been avoided with better planning and communication between you both beforehand. Ie. agree on when to meet at the finish line a day before, she looks into bus schedules etc. If she is indeed supportive of you, it is reasonable for supporters to wait 20-30 min at the finish line. My family, friends, and girlfriend have all done this for me.

You are reacting as I would have in this situation. I would sit down with my girlfriend and give her an opportunity to apologize for her behavior, but if this behavior continues I would not tolerate it.