Frame not suitable for 70" fixed gear?

i run my singlespeed 52/14,i rode it close on 50 miles today and my frame dont creak at all,but my knees do.route today was cycle streatham to victoria train to birchington on sea,cycle along the coastal trail to dover .then train dover to victoria ,then cycle victoria to streatham

you should HTFU or TTFU

...or just use public transport to take you to hill and traffic-free cycle paths, eh?