If it's only a little longer, you should be able to give the Walker a quick charge during a break. 30 minutes on a fast charge will replenish 50% battery.
If it is much longer, Walker is able to charge faster than it uses battery. It is possible to plug in a USB powerbank while you are photographing, and continue working as long as the battery bank allows. Or you can keep swapping out the battery bank in order to continue shooting forever. But, you need to make sure that the battery bank you are using is able to supply at least 2Amps on the port which you are using for Walker.
We actually made a battery holder so that you can keep shooting while charging. It, along with compatible battery banks are available in our store.