Certainly you can substitute other modalities of aerobic training that may be easier on your achilles especially in the early weeks of the training plan. As your goal date gets closer the more sport specific the activity the better. Rowing and cycling are the least ideal as they arent truly weight bearing BUT they might also aggravate your injury the least. I would do some gentle experimentation to see what works. I’ve also had some luck with folks using the stair stepper machine instead of hiking hills or incline treadmill when coming back from achilles issues.