don't worry, stressyhead, it's very difficult to get this sort of thing to the state where you're happy with it.
i would emphasise the point that it isn't what you signed up for and anyway someone else can do it some of the time.
the first para sounds as though you are apologising for failing in your role - do you think this is the case? even if it is, i wouldn't put it in a letter.
if anything it might be worth saying something like...
"I have displayed a great deal of commitment to my role in recent weeks/months by being prepared to travel far in excess of what was agreed with XX.
However, from now on I would prefer to delegate some of these meetings to AE, as he is capable and it would contribute to his development.
This would also enable me to use the additional time I would gain by not travelling to do instead.
In short I believe my proposal has the following advantages .
I believe AE should take over x meetings per month starting from January."
... or something.
hth.