Backpack or rucksack (California DE22)

Looking for a large school backpack or small rucksack that anyone may want to give for rucking - I put weights in the bag and then go for 4,5 mile walks to stay fit and my current backpack has had one of its shoulder straps snap off with wear and tear. Don't want to spend good money on something that people may want to get rid of especially school leavers. Thank you