Well, a good spot to train would be right outside of Russet, where you can fight Pokemon around Lv. 24. But if you have a Pokemon above Lv. 50 with Secret Power, you could train in the secret area that's within one of the Gyms, as it has wild Pokemon that are stronger than anything else you could currently find. If you do decide to start training, try to take it slow and consistent (like one or two levels a day) so you can develop a routine and don't get tired of it