You can start when she is about 5 months old, exactly the age when you shouldn't worry that you can wear her out. There is another reason why that late. Dogs often misunderstand, that they should chase only your bike, not ANY bike, and it is difficult to train them out of it. Puppies with experience of chasing the bicycle run away from their owners only they see one. It would be the end, the whole training with bikes will be out of question. Train her the attitudes, and you can start training her now.
Jogging yourself slowly in the area where people ride, with her wearing harness and on the short lead, train her to give way to other riding bicycle people, train to avoid any collision by stepping aside 3-4 feet and asking her to sit. Think about getting a long rigid plastic stick of light weight with attached loop for your left wrist and carabiner to clasp at her harness to train keeping distance between you and her. You can start walking her on it prior to riding. Please, remember, that you can ride only in the woods, never ride on asphalt paved road, which will not only grind her paws and make them bleeding, but also is very bad for joints. Actually it applies to people as well. Stones, twigs and tree roots are of no problem for the dog, don't worry, you can exercise her by riding 2-3 miles continuously, at 5 months your pup should be strong enough, if you exercised her before. Good luck!