Merkavar

First time posting, planning on using the c25k app but have a few questions. Is it recommended to start from the start or can you skip to say week 4 or 5 if you can already run a bit. On a treadmill I can go for 1.5-2km at 9.5km/h so any reason to not skip a head and give say week 4 a go and see if I can complete it? Speaking of treadmills, can I use this app running on a treadmill? I saw a few threads talking about treadmills so I assume yes. Running out side seems like a barrier for me. Also speed wise, on a treadmill or outside what sort of speed is considered walking, jogging, running for some one 6ft or 183cm or there abouts.