Charles Ellicott Commentary Isaiah 58:8

"Then shall thy light break forth as the morning, and thy healing shall spring forth speedily; and thy righteousness shall go before thee; the glory of Jehovah shall by thy rearward." — Isaiah 58:8 (ASV)

Then shall thy light ... —The dawning of a new day, as in 2 Samuel 23:4; the growth as of new and healthy flesh after long illness; “righteousness,” i.e., the sentence of acquittal in the eyes of all the world, as leading the van of a triumphant march, the “glory of Jehovah” following in the rear as a protection.

All these images are heaped together to paint the fullness of blessing that follows from that true renunciation of the old evil selfishness of which fasting is but a symbol and a part.