I'm not the fittest person but I'm mid-20s, female and had cycled 7 miles the other day (mostly a flat route with some slight uphill and downhill bits). I know it depends on the person but if you're reasonably fit, I'd like your view on the 7 mile distance.
I really admire people who can do like the London to Brighton or even London to Paris cycle routes.. there's no way I would manage that. I don't know how just normal people do it?!