I think the ideal solution, which ok isnt much help for May, is just to run more! (not immediatly but build it up over time) Just get your body used to running for 60+ , 90+ mins. Id also maybe look at ditching the stability shoes and slowly start using something lighter, or alternate between 3-4 different pairs. But to be honest a marathons gonna hurt a little, 26 miles is quite far.
PBs: 5km 18.31, 10k 38:59, HM 1.25.54, Marathon 3.20.17