Well, we can go about it logically. Mauville is on the coast near the center of the landmass of Hoenn. Verdanturf is in the shadow of Mt. Chimney, much further inland, to the point that the soil and land shifts dramatically. If we keep Rusturf Tunnel a reasonable length (10km or less) I would say it's a standard route between Mauville and Verdanturf, at least 30km. A full day's travel on foot.

What I would recommend would be to make the Day Care center its own little village closer to Mauville (let's say 8km), with Bed and Breakfast-style places for people and the Day Care center for Pokemon.

Then, further along you start to see the massive Hoenn Gardens in the rich volcanic soil, and finally, in the shadow of the mountains, Verdanturf, which is where the Gardens are administrated from, etc, etc.