Doing 20 miles in one day might seem easy... but doing 20 miles a day for a month is completely different. The quality of the bike, in particular the seat will also have an effect - you would both definatly have to put some training into it first to reduce injury.
Oh and they have bloody big hills in Scotland, after a couple of weeks of cycling they will seem even worse!
20 miles a day is a very slow pace though isn't it? We did better than that walking the West Highland & the Southern Upland ways.
