Just measured this (The Cumbria Traverse) on Memory Map - it gives 45Km with 3580m of ascent. I find MM slightly estimates distance by between 5% & 10% but is usually pretty spot on with height gain.
Let's say 48Km and 3600m (30 miles and 12000ft in old money) unless John took a GPS with him and has different values.
The only one who can tell you "You can't" is you. And you don't have to listen.