It kind of depends on how much of a detour someone is willing to do for a bite. If you want to stick strictly to the route, and start from Land’s End, there’s pretty much nothing to eat until you hit Park Presidio. If you don’t mind deviating a bit from the route, there’s tons of stuff on that stretch of Clement between 33rd and Park Presidio.
The stretch between 16th/Irving and Glen Park BART is entirely residential, except for that little shopping center where Dewey/Woodside/Laguna Honda meet near Forest Hill Muni station.
Places within 1-2 blocks of the trail (I’m mostly only including places I’ve tried):
House of Bagels
Great Mongolia Nomads
Dong Bei Mama
Wooden Charcoal BBQ
My Tofu House
Pho Huynh Hiep 2/Kevin’s Noodle House
Taco Shop at Underdog’s
Pineapple King Bakery
Hole in the Wall Pizza
King of Noodles
(Haven’t been to any of the places in that Forest Hill shopping center, although My Pot has good Yelp reviews)
Pebbles Cafe (have not tried, bills itself as a “Brazilian Cafe”, could be interesting)
Looks like there are a few places near the Visitacion Valley stretch of the trail. AFAIK there is nowhere to eat from that point all the way to the tip of Candlestick Point, unless you arrange for friends to show up with stuff to grill in the park there.