Travelling by train for this is easy. Marylebone to High Wycombe takes between 30-50 minutes. Tickets on the day are £15 return for adults, or £10 with a railcard discount/under 16's. From the station it's about 3 miles so a bus/cab is required. Unless you are ultra fit like me, to walk it takes around 75 minutes.
I've seen car boot sales with better parking facilities Only joshing. Parking around the Valley is a dark art, and that's if you know all the little back roads. Local knowledge is essential. Still, at least our ground is only 5 minutes walk from the station. I'm sure Wycombe's ground is a splendid venue I don't get to many away games though. I now live so far from Charlton that even home games are a 3+hour round trip...