Can see where you're coming from in some respects, but have a look through the calender and i think there are chances for those between 16-18 to race for 40-50 mins even upto an hour depending on your ability. Obviously you've seen the welsh 1000 which is even longer 2 hours ish. But look at Lingmell dash jebbys record 45 mins you can do that at 16, dalehead same sort of time, dunnerdale later on is a good 50 mins. You've got plenty of time once over 18 to do lakeland classics if you are very keen now why not got some maps from pete bland sports and start reeceing them so you are fully prepared once old enough.