It would depend on where you are in
Disneyland...if you were, say, on Main Street, you'd pretty much have a straight shot down Main Street, through the Park exit turnstiles, across the Esplanade, and about halfway into DTD to get to Tortilla Jo's. So, it'd take about 10 minutes, walking at a brisk pace. If you started from another point in the Park, say, towards the back near Toontown and it's a small world, I'd tack on an additional 15 to 20 minutes, walking at an average pace.
Theoretically, you could also ride the Monorail from Tomorrowland to DTD, and then walk to the restaurant from there. However...sometimes the Monorail breaks down, and sometimes there's quite a crowd waiting to get on. If it were me, and I wanted to take the Monorail rather than walk, I'd allow at least half an hour before the scheduled meal time to wait for/board/ride the Monorail to DTD. I say this because it'd be better to have a few extra minutes to look around DTD before going to eat than being late for & possibly losing the PS reservation.