Have you considered the underground at Amersham? Amersham picks up the overground and you can often get a seat on the direct 7.31 to Marylebone. This gets you in in about 30 minutes. It's also within zone 9, so you get the added benefit of cheaper TFL rates rather than Chiltern Trains prices. It's also on the Metropolitan line if that's more convenient for your commute. Slightly slower but it's the first/ last stop so a seat is guaranteed.