I would just do the Baby Wipe and change clothes. Just easier.

There are plenty of places to get food around the train station. This is a tourist area. It's also down town SD, so lots of food places nearby - but depends on how you feel. If tire and sore you may not be wanting to walk around much. SO I would bring your snacks.

As to the train, well, that's a "crap shoot". Last year the trains did not show up until almost 7. I got in early, around 2:30, it was 100 degrees and I didn't feel well (I rode really hard. Left Irvine at 7:45 and got in at 2:30. A personal best for an Century). Last thing I wanted to do was sit and wait for a train. The station was not air conditioned. It's really old!

The train will be packed. Some cars will be loud, noisy and obnoxious. If that's your cup of tea look for the BCI/OCW or Arizona folks and board with them. I try and avoid the noisy groups so usually go to the front cars away from the party people. That way I can have a nice conversation with the folks around me. The ride last year only took about an hour but we were the last train for the evening and there were no stops. The 4:30 train will stop in Del Mar, San Clemente, San Juan etc.

If you tend to get cold make sure you bring a jacket or something to put on while waiting at the station. SInce you will be right on the water, it can be cold (although like I said last year it was 100 blasting degrees!).