How to plan a route

Other people who lead our rides may have other solutions to this challenge, but I thought I would explain what I do.
I start with an idea - e.g. "we've not been to Musselburgh for a while" or "I'd like to go to that new Climbing Arena at Ratho".
This website here: www.bikehike.co.uk is an excellent place to start planning the route. You can see Ordnance Survey maps and Google maps (satellite view and normal map view), and there is also an overlay which shows lots of cycle paths (and footpaths) around the city. Click on the OSM cycle button to see what I mean. With the map zoomed out to see the whole area, one gets an idea for the broad plan for the route.
After going out with the group for nearly 3 years, I would also say that I am quite familiar with the "main" routes around the city - e.g. the Innocent cycle path route to the East, or Route 1 out towards South Queensferry, or the Canal / Water of Leith to the South, or the Roseburn Park/Corstorphine route to the West. So if I want to get outside the bypass on the ride, it's probably best to take one of these routes, at least in one direction, as they are easy to navigate and generally quiet.
I then work out some details: as far as possible I do this on the map; try to work out difficult junctions, or spot busy roads. Sometimes the StreetView on google maps is a great help for actually seeing what a road looks like (jogging my memory). Street View doesn't apply on paths though, so it's only of limited use. When I'm travelling around town at other times I keep my eye out for paths I haven't seen before - a bit of park, a cycle sign or some new area that's opened up. It's also important to remember that the best bike routes are ones that one can't see from a car or a bus.
The final planning stage is to to a recce - preferably not too long before the day of the group ride, as otherwise problems like road works or path closures can happen, resulting in detours. On the recce I try to follow the map route I've worked out, but also see if there is anything interesting I've missed that we could do instead. Sometimes, what looks great on the map is absolutely horrible in the real world - a steep hill, a ploughed field, a quagmire of a path. There have been times when I've torn the whole route up at that point and started again. Or decided to run the route anti-clockwise instead of clockwise, to avoid a steep climb or steps. So it's important not to leave the recce to the Very last minute, as this would mean there was no time to re-recce the new route.
The distance is important to consider - anything from 16 miles or so in the winter (short days, bad weather) to 25-ish in the summer is acceptable, as long as the route is interesting and - most importantly - there is a nice place for lunch around 12.30 - 1pm, that can cope with the arrival of 15-35 hungry cyclists all at once, with somewhere safe to lock up bikes and somewhere that people can also have picnics should they wish.
It has been known to have two routes prepared - for example in October when the forecast was unpredictable (potential high winds) Andrew had an alternative route planned that would stay within the city limits for shelter (and also this gives people the opportunity to give up and go home much more safely than if we were 10 miles away at the airport if the weather became uncyclable-in). In the event we didn't need it, but it can always be used for a future ride, once it's ready!