It's a long, tough, challenging walk. Personally I would recommend you take your mate's advice and get some good training walks in on similar terrain. Start with a mile or 2 and work up to it. Get some biggish hills in too. I would aim to be comfortable with doing around 15 miles or more over similar ground before doing this.
I'm sure that many of those that do it every year do so with little to no training. I'm equally sure that many of those have an awful day out as a result.
A couple of years back, and mate and I were climbing up bracken bottom towards Pen y Ghent. We passed a young lass on a charity outing just as P-Y-G hove into view (maybe a mile in) Bl00dy hell! she exclaimed. "I hope we're not going up that!!" She was clearly totally unprepared for what was ahead. I didn't have the heart to tell her that indeed she was "going up that".
All that said, it's a beautiful walk in stunning scenery. I hope you're successful with it, but please get some training in and above all don't underestimate it.
I'll also add the usual caveats here about appropriate clothing, plenty of food and drink and at least reasonable navigation skills.
I'm sure others will be along soon to add to this.