Is there a section that follows King John's crossing of the Wash? If there is I'd avoid that bit.
Although, boringly, if we did know John's actual route we'd probably find it to be perfectly safe now due to land drainage.
And somebody would have found the treasure!
I reckon it's under Cross Keys Bank now so will never be uncovered again. That's a cycleway along the old A17 after the A17 took over the former railway, so that doesn't protect it as such (Lincs CC basically seem to hate cycling infrastructure), but even if government was to widen the A17 (why? It's way below capacity), they'd widen onto the flat land to its south instead of expensively dismantling an old bank and risking uncovering something surprising.
Shouldn't the King John Way visit his statue, erected in King's Lynn by some former council leaders who apparently felt inspired by the worst king ever 🤣 ? If it did, it would link up with the Rebellion Way, which links to the Wolf Way and Ickneild Way Cycle Trail, opening up a massive bikepacking trip across east and central-south England.
If you want to add a pilgrimage to Johnny's effigy at the entrance to an ugly modern shopping centre, head east from Holbeach on the B1515 and B1359, cross Sutton Bridge, then follow blue arrows along the former A17 ('orange' local cycle route) to King's Lynn!