Light Fare/Lunch between Train and Plaza Armas Bus

Most large train stations have plenty of food operations, especially for sandwiches and snack items.

I zoomed in on a Google Map showing the walking route, and it shows dozens of restaurants and cafes right on your way, so you will have many options. Google estimates the walking time at 31 minutes, and I'd expect it to be longer if you are pulling a suitcase or wearing a full-size backpack.

In case you are not aware, Spanish meal hours are quite late; lunch could be 1:30 till 4 PM or something like that. Earlier than that, and a nice carryout spot may be your best bet.

Don't wait until you get to the bus station. It is very basic and had very few shops when we passed through in March.

Sevilla Santa Justa’s station has several places where you can grab something to eat, including a McDonald’s.
Here’s the list of eateries at this station: Estaciones.Adif.es

Enjoy your trip!

Ate lunch in one of the cafeterias at Santa Justa on our last trip (1 hour layover on the way between Granada and Cadiz), and it was.... edible. So, it's an option if you're truly just looking to fill up, but.....

If it were me in your situation, I'd look on google for a place closer to the bus station and take a taxi from Santa Justa to the restaurant. Grab a meal and then do a shorter walk to the bus station. Agree with AussieNomad, there's not much at Plaze de Armas, so make sure you eat before you get there.

Thanks for the suggestions. Ended up deciding to wait til destination to eat. Have to warn everyone the bus station at Plaza Armas is disgusting. The bathrooms filthy no toilet paper or seats for that matter. Saw McDonalds express window from arriving bus 21 we ended up taking from train station to bus station however McDonalds was not accessible once inside bus station. There also were lots of people begging even on the bus platforms. Seville should do better. Other smaller cities in Spain do.