I assume you already do some cycling and swimming if you want to do a triathlon? Those will help build your cardio base. Along with strength training specific for running that should help you get back faster.
For cycling, check the position of your cleats are good. I think having your feet a bit more to the front on the pedals (so cleats more towards the centre of the foot) can help spare your calve muscles and focus cycling more on the upper leg, which helps them stay fresh for running and avoiding tight calves.