fat old men are frequently seen frolickign up there - the main key i think is just establishing a good pace - if, at any altitude, you can hold your heart rate at 70-80% of your max for 50-60 minutes at a time (w/ a 10 min break or so afterwards b4 you go again), you ought to be fine and actually enjoy it (unless of course it's bitterly cold, windy, foggy, crevassy, etc. etc. )
key is, don't go in a huge-cattle train where you have to fit an unnatural pace - slow n' steady n' take a bunch of pictures