You don't need to jump the barrier, nor catch a bus. There is a foot tunnel UNDER the Highway - so all you need to do is 'retire' at about 13-14 miles, cross under the road, then rejoin the race after you've "had a pee".
I think cycling will have to wait until I can balance properly - I've a tendency to fall to the right at the moment! At least with walking or running you have some opportunity to correct yourself. Bikes are less forgiving.