70 miles on a fixed is no biggie if you are in shape and your gearing is right for the terrain, especially if you have a flipflop with a bailout freewheel. Given that your friends are non cyclists as you say you should have no trouble keeping up with them. If anything they will be the ones who will be suffering. But why not just go out on your fixed bike for some longer rides and see how you do? I think you'll be surprised.