This, then, is only a fault of your metric. If you want to measure endurance you can use different means of measurement as suggested in the other child post. Especially in the case of inability to walk you can measure endurance with a wheelchair. Running is just one of many options to measure endurance.

This is plausibly related to a coordination disability, though. I do okay at other measures of endurance that don't require coordination. My suggested solution is to have either multiple options at each level (run one mile or walk three miles without stopping) or use different exercises for the different benchmarks (run 1 mile for level 1, walk 10 miles for level 2) .