You need to be more clear Georgie.

Cardio = cardiovascular fitness. So yes, it would work but it would be limited.
I'm
guessing you meant overall endurance. Which yes, it would slowly. But pretty soon you wouldn't be doing 1/2 your maximum, you'll be doing 1/3 without realising it! So you'd have to slowly up mileage and test max regularly (every few weeks).