Advice needed for charity ride

[simonhill]

My suggestion: you supply a bit more info.

You don't need to apologise for being new, we all start sometime, but if the replies are going to be targeted and appropriate then more info is needed.

A brief description of your plan - one day, solo, unsupported, etc.
A brief description of you - fitness, cycling experience, etc
A brief description of your bike.

[PH]

If you're looking for a group to ride with and build up the miles, then South Bucks CTC (The local Cycling UK member group) is probably a good place to start. I've ridden with them a few times and they're a very friendly bunch and welcome new riders.
http://www.southbuckscycling.org.uk/

I've also ridden Derby to that area and back several times (For South Bucks CTC Golden Beeches Weekends) I've done a different route each time. There's no need to use any busy A road or poor quality off road facilities, happy to share routes if you give some more detail.

Route 6 or 52 have been suggested.

Don't remember if I've done any of 52, but 50 might also suite you and it's all on quiet lanes (No off road) though you do go through the centre of Daventry. If you put the two places in cycletravel, that's the way it'll take you. Looks a decent enough route, though you can always tweak it to you preferences.
http://cycle.travel/map/journey/58797