It was good fun, and the Panda handled it well. I always get a little disconcerted when I hear rocks hitting the underbelly - I need to get it on a ramp to understand any points of vulnerability. Saying that, I did have a Defender a few years ago and I felt the same way about that vehicle.
It was a bit of a last minute decision to do this, and I probably spent a couple of hours researching on the web. Living in Lancashire, like many areas of the UK, you quickly find that green lanes either do not exist, or are not documented anywhere accessible. Cumbria County Council, on the other hand, are very good at publicising the routes AND educating on correct use. For example, I found one route that should have been passable, had a river crossing, but also had Salmon spawning - it didn't say the route was closed at this time of year, just that you should consider this and your potential impact. I skipped this route. I also kept to the dedicated tracks to avoid churning up the already sodden grass either side.
So, a good start point is:
www.cumbria.gov.uk/roads-transport/...de-access/hotr/hierarchy_of_trails_routes.asp
This page provides an explanation of the hierarchy of routes (Red, amber & green). From this page you can then access different OS areas - I went for OS97. On that page you can get an overview of the routes for the east and west. These overview maps are very useful for planning your day as you can try and create an overall circular plan. The routes should are colour coded - I chose the red and amber routes. Once you have a plan, each route then has it's own detail page which gives an OS reference for the start and end, general descriptions of the surface (grass, crushed stone, rock), and any notes of warning e.g. river crossing, livestock etc.
I took a copy of an OS map on my iPad, and it was useful in identifying the start points. Note that this is probably the trickiest part of the day. However, many will have a subtle signpost to indicate the start. Don't be put off by gates, you have the right to access!
I did come across one red route which I managed just 100 metres of, finding it quickly turned to large rocks. But two other red routes were perfectly passable. Some of the Ambers were very easy, but wonderful all the same.
Having now tried green lanes I will definitely do it again, maybe quite regularly. They do not over-challenge you as a driver, they are more about getting out into the open countryside and enjoying it, without interfering with it. I took my kids and they also enjoyed themselves, at the pub lunch at the end. I recommend it, and again, thanks to Cumbria County Council.