I have ridden across Bulgaria on my Brompton, a lot of it off road. I could have done it more comfortably on my MTB or gravel bike and probably a lot faster. However, getting the Brompton on trains, buses, ferries, aeroplanes is far easier that a DF bike. Riding long distances on it is not a problem at all.
Its horses for courses. You are not going to keep up with the peleton on the Brompton. But with the gearing on my 6 speed Brompton, I can get a really good workout, if thats what I want to do.
I have 8 bikes. It is probably 5 bikes too many. Going down the multiple bike route is a very slippery slope as many people on here will tell you.
You do not need another bike. You just want one.
