the Ingles is 120km or so long. One of the attractions of the Ingles is that you can do the whole route. As said in posts above, there is no rule that you must start at Ferrol harbour, but on the basis that most people will stay in Ferrol the night before they start, not starting at the start, and taking a picture of the waymark which commemorates the start of the Ingles; i.e. the historic point at which pilgrims came out of their boats at the harbour and started to walk to Santiago, for me, not starting at the start at the harbour seems to me to miss something.
One possible reason not to start at the start at the harbour could be that if you are walking the whole first stage to Pontedeume, it is long - 29km or so, and starting from your hotel in Ferrol will knock 2km or so off the first day.
This is true, but as Tia writes, there is nothing to stop you walking down to the harbour the afternoon / evening you arrive in Ferrol and walking the Ingles to your hotel / level with your hotel and then next morning walking straight out your hotel and carrying on from that point. This is exactly what I did, and also obtained my first stamp at the tourist office by the harbour, stamped for next day, if you wish, because the tourist office does not open until 10am, far later than most have commenced walking.
Also, if the first day per the guidebook seems too long, consider walking to Xubia or Neda and staying overnight there, so that you do not overtire yourself on the first day. Consider also taking the short cut either over the N651 road bridge to Fene, or over the railway bridge to Xubia (assuming the repairs have been completed and the footbridge is open). These short cuts knock up to 10km off the first day, and you still walk 100km to Santiago and so can qualify for a compostela.
But I would start at the start, even, if you do this the evening before. For me, you miss something if you don't.