Did about 10.5 km today in 50 minutes I know not great compared to some but on a small Japanese folder its not bad going. I think I could maybe do 15 km but that would be it regarding comfort. I am not trying to prepare for anything just trying to maintain my fitness and hopefully improve it. I incorporated some sprints into my ride today and felt the benefit, even on a small bike you can get something out of your ride besides site seeing or just enjoying the scenery.
I am hoping to start going to the gym soon to supplement my riding with a bit of strength work in preparation for returning to the UK where I would like to take part in some CX events during the Autumn/Winter, I need to get a new bike for sure but I have found it really does not matter what you ride just as long as you ride!